score:8
But why would Germany even want to keep a Hitler-issued law in effect at all let alone so long after ww2 ended?
Because the Basic Law (Constitution) Article 16(1) does not generally allow citizenship to be revoked. This provision was introduced as a result of the wide spread misuse during the period between 1933–45.
Article 16(2), as it was until 2000 when the second sentance was added, did not permit the extradition of German citizens to other countries.
Article 16 [Citizenship - Extradition] - Basic Law for the Federal Republic of Germany
(1) No German may be deprived of his citizenship. Loss of citizenship may occur only pursuant to a law and, if it occurs against the will of the person affected, only if he does not become stateless as a result.
(2) No German may be extradited to a foreign country.
Based on the Decree of 1943-05-19 (PDF):
(1) Deutschstämmige Ausländer, die der deutschen Wehrmacht, der Waffen-SS, der deutschen Polizei oder der Organisation Todt angehören, erwerben mit der Verkündung dieses Erlasses die deutsche Staatsangehörigkeit.
(1) Foreigners of German origin who belong to the German Wehrmacht, the Waffen-SS, the German police or the Organization Todt acquire German citizenship upon the promulgation of this decree.
...
Heinrich Boere, who was born in Germany of a Dutch father and a German mother, was a German citizen when the Basic Law came into effect on the 25th of May 1949.
Your quoted article also states:
According to Dutch law, being convicted of serving the army of the enemy automatically leads to the loss of Dutch citizenship.
so even if the the Decree of 1943-05-19 had been nullified after the 25th of May 1949, the second sentance of Article 16(1) would have prevented the loss of citizenship.