Why was there a "workers' militia" in Eastern Europe but not in the USSR?

This seems to be more of a language problem than anything else.

The Soviet Union would not impose these detailed requirements for getting a state from socialism to communism. While one might argue that Eastern European countries were somehow colonised by the USSR, one cannot argue that they were required to copy every detail the Soviet Union served as a an example for.

1. The Soviet Union in its infancy had organisations that were like the Worker's Militia or the Combat Groups of the Working Class. In 1917 there were two of them the 'Workers and Peasants' militia' and the more closely related to their later GDR counterpart, called 'Workers' Militia'. They dissolved into the Red Guards and thus the Red Army.

   It is quite irrelevant how any armed force of oppression in any state is called. Whether it's for some "operation legend" or "workers combat groups", the central fact is just that these are para-military groups.
   In the GDR the precursor for the regular army National People's Army was called "barracked people's police" and the regular police in the Soviet Union was called the Militia. Having an official para-military might have some advantages, which is irrelevant to argue here. But being official they need to be well-regulated, Sadly, that's not always the case.

2. The reasons for establishing the named para-military groups in Eastern Europe were just this: to increase the numbers of loyalists for 'self-defense' of the workers' paradises. Against saboteurs, counter-revolutionaries and spies. In other words: to have more men at the disposal to quell any rebellions possible. As such the USSR did just fine with their police, KGB (use the appropriate acronym for the desired time-frame) and if need be, they too and for sure would have mobilised and deployed Red Army units.

3. An equivalent for the Eastern European combat groups and quite comparable, albeit even with planes at their disposal, was present in the USSR:

   DOSAAF (Russian: ДОСААФ), full name Volunteer Society for Cooperation with the Army, Aviation, and Navy (Russian: Добровольное Общество Содействия Армии, Авиации и Флоту), was a paramilitary sport organisation in the Soviet Union, concerned mainly with weapons, automobiles and aviation. The society was established in 1927 as OSOAVIAKhIM and from 1951 to 1991 carried the name of DOSAAF.

   The society was preserved in some post-Soviet Republics, e.g. Russia and Belarus, although these may use a different name. In Ukraine, for example, the counterpart is "Society of Assistance to Defense of Ukraine". In Russia it was reformed in December 1991 as the Russian Defence Sports-Technical Organization ROSTO (Russian: Российская оборонная спортивно-техническая организация – РОСТО). In December 2009, ROSTO was renamed DOSAAF Russia.1 For Belarus, see DOSAAF (Belarus).

If we restrict ourselves to one universal definition of 'militia', mainly a 'people's army for defence', ignoring those plays on words we find when the word 'militia'is used in slightly or completely other contexts, then we see a slightly different picture emerging:

Militias thus can be either military or paramilitary, depending on the instance. Some of the contexts in which the term "militia" can apply include:

• forces engaged in a defense activity or service, to protect a community, its territory, property, and laws,
• the entire able-bodied population of a community, town, county, or state available to be called to arms
• a subset of these who may be legally penalized for failing to respond to a call-up
• a subset of these who actually respond to a call-up regardless of legal obligation
• a private (non-governmental) force not necessarily directly supported or sanctioned by its government
• an irregular armed force that enables its leader to exercise military, economic, or political control over a subnational territory within a sovereign state
• an official reserve army composed of citizen soldiers known as the militsiya,
• a select militia composed of a small, non-representative portion of the population,
• maritime militias composed of fishermen and other participants of the marine industry which are organized and sanctioned by the state to enforce its maritime boundaries.

For these highlighted reasons, the Eastern Bloc showed these characteristics:

At all times the left-wing workers' movement preferred the militia concept in the event of an uprising (revolution) or the defence of existing workers' and peasant states. The reasons were:

First, the political influence of the ruling order was high on the existing (professional) armies, which were an integral part of the valid order. The formation of an alternative military potential had to be recruited almost entirely from the population and naturally led to militias through general armament and shorter training periods. Secondly, the political control of the democratic institutions (Workers' and Soldiers' Council), independent of a politically dominant party over the military units, was to remain strengthened and protect the soviet republic or, in the case of bureaucratic development and political instrumentalization, maintain the power position of the population (political revolution).

The first militia in the modern sense of the word came into being at the end of the Franco-Prussian War, when on 18 March 1871 the Parisian population, together with the republican-oriented National Guard, opposed disarmament by the Emperor Napoleon III, who collaborated with the Germans, and his conservative-royalist central government under Adolphe Thiers, proclaimed popular armament, fortified positions in Paris and called new elections. This was the birth of the short-lived Paris Commune.

In the revolutions of 1917-1923, the insurgents' military forces consisted of militias and associations of armed workers:

Red Guards (1917-1918), the basis of the Red Army created in 1918 Red Ruhr Army (1920), reaction to the Kapp Putsch. Exceptions were mutinous, deserting or defected regular units such as the Kronstadt sailors and fortress garrison or the People's Navy Division.

In the Soviet Union, the concept of militias or territorial armies was abolished with the reform of the armed forces in 1935.[…] After the Second World War, in the course of the bloc confrontation and in view of the danger of war in the Eastern bloc, systematically standing armies with an additional conscription reserve were built up. However, the militia system continued to exist in part, for example in the form of workers' militias in the GDR such as the Betriebskampfgruppen (1952-1990) or in the People's Republic of Poland (1944-1990) in the police force known as the Bürgermiliz. — deWP: Miliz (Volksheer)

The above is repeated in an analyses of the GDR Kampfgruppen close to their dissolution:

In a situation like that of the autumn of 1989, where colleagues and friends who took part in the demonstrations were presented as ‘enemies’ who posed no other option than to be ‘crushed’, militia members began to realize that the ‘last revolutionaries’ clung to an understanding of politics that had taken shape in the crisis-ridden period between 1918 and 1945 and was based essentially on militant action – politics thus always had to be considered as a matter of life and death. While this seems somewhat contradictory to the ubiquitous Socialist rhetoric of ‘peace’ and ‘harmony’, it was precisely ‘the regime’s stubborn denials of the existence of any social conflict’ that ‘made elementary conflict into an existential threat’, as Steven Kotkin has written about in Uncivil Society. A ‘latent risk of upheaval’ that characterized Socialist statehood along these lines was constantly reproduced by its leadership, who were unable to consider ‘their’ state as an end in itself (even after forty years of Socialist rule). Furthermore, considering themselves as being in the vanguard of Communism, Dickel’s reference to the 1953 uprising shows that they never overcame a strong feeling of distrust towards ‘their’ populace.

Along these lines, more and more Factory Combat Unit members in 1989 stated that they did not want to act as the 'club guard' (Knüppelgarde) of the SED, respectively the state’. This contradictory self-depiction seems revealing both with regard to the nature of Socialist statehood and with regard to the status of militia members. While in theory there was no difference between the party and the populace, protecting the Socialist achievements meant protecting the party’s cause. Factory Combat Unit members were thus dedicated to the Socialist populace as much as they were to the party. The insecurity voiced above about acting on behalf of the party or the state, respectively its populace, indicates that up until the autumn of 1989 both were thought of as synonymous but fell apart in the face of mass protests. The political, hitherto thought of primarily in terms of order, now regained its nuances. The GDR was never ruled by politicians serving a state that was considered an end in itself, but by ‘revolutionaries’ who had occupied the state, using it for their cause and striking against everyone who dared to oppose it. Many members of the militia considered restoring order and safety a just obligation, but using violence on behalf of a party that obviously denied the concerns of its (peaceful) followers did not seem a legitimate option.

The more the SED insisted on dealing with ruthless ‘class enemies’ – ‘It’s D-day today,’ members of a police unit were told by their officers, ‘it’s either them or us’ – the more members of the armed forces, including Factory Combat Group members, became disillusioned with their leadership:

We had presumed that there weren’t only individual fighters, individual production workers, but functionaries and members of the SED’s district council as well. Of these Comrades, no one was to be seen. We thought of ourselves as being a bunch of misery expected to ‘save the Republic’.

Eric Weitz has stated that ‘by the end of the 1980s, the GDR population had grown intensely weary with a political language and political policies rooted in the 1920s and increasingly removed from the concerns and realities and desires of everyday existence’. No other institution made that plainer than the Factory Combat Groups. Rooted in the 1920s and aimed at reviving the spirit of the Red Front Fighters’ League, this militia represented the idea that Socialist politics were essentially combat politics, and always had to anticipate the ‘state of emergency’. Because the GDR rulers clung to a militaristic jargon that reflected their self-depiction as ‘revolutionaries’ rather than ‘politicians’, this rhetoric finally served as eventual proof for the permanent exclusion of the real people by those who held power – with the ultimate consequence that the regime lost all credibility.

When the SED aimed for a ‘Wende’, one of the first things it decided besides changing its name to Party of Democratic Socialism (PDS, Partei des demokratischen Sozialismus) to demonstrate its renewal symbolically was to dissolve the Factory Combat Units. For the designated chair of the ‘new’ party, Gregor Gysi, the future of the Factory Combat Units was among the five major issues to be dealt with at an emergency party conference in December. Although Gysi showed respect for the commitment of the militia, he made clear that the Factory Combat Units were ‘outdated’. Strikingly, his statement met no opposition. The organization upon which the ‘last revolutionaries’ had prided themselves was considered no more than a long-lasting (self-) deception after ‘their’ state had ceased to exist.
_{— Tilmann Siebeneichner: "‘Socialist achievements’ and their protection. The Kampfgruppen der Arbeiterklasse and socialist statehood", Social History, 39:4, 462-488, 2014. DOI.}

For an interesting comparison of how the 'sword and shield of the party', – the Stasi, greatly expanded after the uprising of 1953 – view the added numbers, loyalty, motivation and capabilities of the Worker's combat groups at the time, read this internal document produced around the 40th birthday of the socialist state Germany. The directly controlled and also quite well armed Stasi was always much more important, more tightly organised, more motivated and more unquestioningly loyal to the state it served.

Since the issue of trust came up in comments: it is still a problem of language, naming and organisational structure. 'Worker's combat units' are an official state paramilitary grouping, as found in Eastern Europe. If you look at the Internal Troops of the USSR, things looks very much the same, except on an even higher level of direct ministerial state control and organisational naming or 'placing' within the hierarchies. Compare the following with how the US National Guard and similar groupings are organised, and for rasons of legality, outward display etc, the names may change a lot, the raison d'etre not so much:

Change of subordination of the Internal Troops On March 15, 1946, the NKVD of the USSR was transformed into the Ministry of Internal Affairs of the USSR .

On January 21, 1947, the Internal Troops of the Ministry of Internal Affairs (operational units) were reassigned to the USSR Ministry of State Security (MGB USSR). The convoy troops remained in the USSR Ministry of Internal Affairs.

On July 10, 1949, the escort units were entrusted with escorting prisoners to judicial institutions, to exchange offices of planned railway routes in republican, regional and regional centers.

On May 6, 1951, by the Resolution of the Council of Ministers of the USSR , convoy guards were entrusted with the transfer of prisoners and those under investigation by planned (special) convoys along railway and waterways, as well as their transfer from prisons to camps and colonies; also, at the request of the Prosecutor's Office and law enforcement agencies, they are entrusted with escorting them to court sessions of the Supreme, regional, regional courts, military tribunals, liner courts - by rail and water transport; convoy to wagons at exchange offices.

By 1957, the number of the Internal Guard was 55,715 people, the Convoy Guard - 33,307 people, and the formed Convoy Guard of places of detention - 100,000 people.

On January 13, 1960, the Council of Ministers of the USSR abolished the Ministry of Internal Affairs of the USSR, transferring its functions to the Ministries of Internal Affairs of the Union republics. Accordingly, the Internal Troops are distributed among the union republics and are transferred to the subordination of the republican Ministry of Internal Affairs on a regional basis.

On August 30, 1962, the Presidium of the Supreme Soviet of the RSFSR reorganized the republican Ministry of Internal Affairs into the Ministry of Public Order of the RSFSR (MOOP RSFSR). The same was done in all union and autonomous republics of the USSR. Internal troops were reassigned to the republican MOOP .

On July 26, 1966, by the Decree of the Presidium of the Supreme Soviet of the USSR , the central body for managing law and order was restored in the form of the Ministry for the Protection of Public Order of the USSR (MOOP USSR). The internal troops are included in the USSR MOOP .

On November 25, 1968, the MOOP of the USSR was renamed the Ministry of Internal Affairs of the USSR. Internal troops again find themselves in the structure of the USSR Ministry of Internal Affairs.

On March 21, 1989, by a decree of the Presidium of the Supreme Soviet of the USSR, the Internal Troops of the Ministry of Internal Affairs of the USSR (along with the Border Troops of the KGB of the USSR and the Railway Troops) were withdrawn from the Armed Forces of the USSR.

By the decree of the President of the RSFSR dated October 20, 1991, all formations of the Internal Troops of the USSR Ministry of Internal Affairs stationed on the territory of the RSFSR were taken under the jurisdiction of the RSFSR and subordinated to the republican Ministry of Internal Affairs.
_{— ruWP: Внутренние войска МВД СССР, section post WWII–1991}