What was the true status of Hessian mercenaries?

The song "Frisch auf, ihr Brüder ans Gewehr" gives it away:

𝔉𝔯𝔦𝔰𝔠𝔥 𝔞𝔲𝔣, 𝔦𝔥𝔯 𝔅𝔯𝔲̈𝔡𝔢𝔯, 𝔦𝔫𝔰 𝔊𝔢𝔴𝔢𝔥𝔯,
'𝔰 𝔤𝔢𝔥𝔱 𝔫𝔞𝔠𝔥 𝔄𝔪𝔢𝔯𝔦𝔨𝔞!
𝔙𝔢𝔯𝔰𝔞𝔪𝔪𝔢𝔩𝔱 𝔦𝔰𝔱 𝔰𝔠𝔥𝔬𝔫 𝔲𝔫𝔰𝔢𝔯 ℌ𝔢𝔢𝔯,
𝔙𝔦𝔳𝔞𝔱 𝔙𝔦𝔨𝔱𝔬𝔯𝔦𝔞!
𝔇𝔞𝔰 𝔯𝔬𝔱𝔢 𝔊𝔬𝔩𝔡, 𝔡𝔞𝔰 𝔯𝔬𝔱𝔢 𝔊𝔬𝔩𝔡,
𝔇𝔞𝔰 𝔨𝔬𝔪𝔪𝔱 𝔪𝔞𝔫 𝔫𝔲𝔯 𝔰𝔬 𝔥𝔢𝔯𝔤𝔢𝔯𝔬𝔩𝔩𝔱.
𝔇𝔞 𝔤𝔦𝔟𝔱‘𝔰 𝔞𝔲𝔠𝔥, 𝔡𝔞 𝔤𝔦𝔟𝔱‘𝔰 𝔞𝔲𝔠𝔥,
𝔇𝔞 𝔤𝔦𝔟𝔱‘𝔰 𝔞𝔲𝔠𝔥 𝔟𝔢𝔰𝔰𝔢𝔯𝔫 𝔖𝔬𝔩𝔡!

𝔇𝔞𝔰 𝔏𝔢𝔟𝔢𝔫 𝔥𝔞̈𝔱𝔱𝔢𝔫 𝔴𝔦𝔯 𝔥𝔦𝔢𝔯 𝔰𝔞𝔱𝔱,
𝔚𝔦𝔯 𝔴𝔬𝔩𝔩𝔢𝔫 𝔦𝔫 𝔡𝔞𝔰 𝔉𝔢𝔩𝔡,
𝔚𝔢𝔦𝔩 𝔪𝔞𝔫 𝔧𝔞 𝔨𝔞𝔲𝔪 𝔷𝔲 𝔣𝔯𝔢𝔰𝔰𝔢𝔫 𝔥𝔞𝔱,
𝔇𝔞𝔷𝔲 𝔰𝔬 𝔴𝔢𝔫𝔦𝔤 𝔊𝔢𝔩𝔡.
𝔘𝔫𝔡 𝔢𝔦𝔫𝔢𝔫 𝔗𝔞𝔤 𝔲𝔫𝔡 𝔞𝔩𝔩𝔢 𝔗𝔞𝔤'
𝔇𝔦𝔢𝔰𝔢𝔩𝔟𝔢 𝔓𝔩𝔞𝔤', 𝔡𝔦𝔢𝔰𝔢𝔩𝔟𝔢 𝔎𝔩𝔞𝔤',
𝔖𝔭𝔦𝔢ß𝔯𝔲𝔱𝔢𝔫, 𝔖𝔭𝔦𝔢ß𝔯𝔲𝔱𝔢𝔫,
𝔖𝔭𝔦𝔢ß𝔯𝔲𝔱𝔢𝔫, 𝔡𝔞𝔰𝔰 𝔢𝔰 𝔨𝔯𝔞𝔠𝔥𝔱.

𝔄𝔡𝔠𝔥𝔬̈, 𝔪𝔢𝔦𝔫 ℌ𝔢𝔰𝔰𝔢𝔫𝔩𝔞𝔫𝔡, 𝔞𝔡𝔠𝔥𝔬̈!
𝔍𝔢𝔱𝔷𝔱 𝔨𝔬𝔪𝔪𝔱 𝔄𝔪𝔢𝔯𝔦𝔨𝔞,
𝔘𝔫𝔡 𝔲𝔫𝔰𝔢𝔯 𝔊𝔩𝔲̈𝔠𝔨 𝔤𝔢𝔥𝔱 𝔦𝔫 𝔡𝔦𝔢 ℌ𝔬̈𝔥',
𝔊𝔬𝔩𝔡𝔟𝔢𝔯𝔤𝔢 𝔰𝔦𝔫𝔡 𝔞𝔩𝔩𝔡𝔞!
𝔇𝔞𝔷𝔲, 𝔡𝔞𝔷𝔲 𝔦𝔫 𝔉𝔢𝔦𝔫𝔡𝔢𝔰𝔩𝔞𝔫𝔡,
𝔚𝔞𝔰 𝔢𝔦𝔫𝔢𝔪 𝔣𝔢𝔥𝔩𝔱, 𝔡𝔞𝔰 𝔫𝔦𝔪𝔪𝔱 𝔡𝔦𝔢 ℌ𝔞𝔫𝔡,
𝔇𝔞𝔰 𝔦𝔰𝔱 𝔢𝔦𝔫, 𝔡𝔞𝔰 𝔦𝔰𝔱 𝔢𝔦𝔫,
𝔇𝔞𝔰 𝔦𝔰𝔱 𝔢𝔦𝔫 𝔞𝔫𝔡𝔯𝔢𝔯 𝔖𝔱𝔞𝔫𝔡

There were a few soldiers pressed into service. Mainly because this was a form of deterrence for preventing desertions: once signed up, willingly, a man had sold his skin and his superiors went not only after him but after his family to compensate losses from desertions. Or the village if no suitable male would be on hand from that family.

But as the song makes clear, most signed up, explicitly to be 'sold'. Or better to go to America, defeat the rebel scum and plunder what they could. Not in the least because the conditions in Hesse was indeed quite bleak. A career in the armed forces as usual seen as an opportunity.

Those that were pressed – or seen as probably pressed, dominated the narrative, like Johann Gottfried Seume: "Mein Leben", Reclam: Stuttgart, 1986:

At the end of June 1781 he set off for France, but already on the third day

the Landgrave of Kassel, the great human mover at the time, took over the procurement of his future night quarters through his advertisers despite all protest.

But even this is contested for its truthfulness, as the nonchalance of this rather cynical witticism might indicate that he enjoyed the thought of going to America.

How many of the Hessians were volitional is exemplified by the low rate of desertion in Germany. Only 13 went missing. And even more so by looking at the plans the American illoyalists had with them:
Pressed mercenaries should be easy to sway into desertion?

^{Handbill probably written by Thomas Jefferson, translated and printed under the supervision of Benjamin Franklin, and distributed among the Hessian troops on Staten Island, August, 1776. From the only known copy, in the German State Archives at Marburg}.
_{–– Lyman H. Butterfield: "Psychological Warfare in 1776: The Jefferson-Franklin Plan to Cause Hessian Desertions", Proceedings of the American Philosophical Society, Vol. 94, No. 3, Studies of Historical Documents in the Library of the American Philosophical Society (Jun. 20, 1950), pp. 233-241. (jstor)}

How many took up that generous offer is "slightly" overestimated by deductive logic with not much base by Butterfield. The real numbers seem to be less than half of his estimate.

Or reversing the perspective:

They were unquestionably better prepared mentally and militarily to confront the rebels than any other auxiliary corps. Unlike the Ansbachers, only half of whom were natives, or the Brunswickers, who counted only 600 natives among their 4,000-man contingent, the original 12,000 Hessian troops dispatched to America were a largely h*m*geneous force that prided themselves in being Landeskinder.

They also compare favorably to the American "citizen army" that opposed them. In many respects the two forces were not much different from each other. Both comprised a combination of social outcasts and the very poor, with almost no tradesmen or landholders. Both also included many "volunteers" who had enlisted because of peer pressure and coercion, as well as former militiamen who had never expected to go to war but who were now obligated to do so. If anything the Hessians were the more representative of the poorer and more militaristic society from which they came. At least in the beginning they also fought much better and with more enthusiasm and were more faithfully and adequately attended to by their own government.

Nor were the Hessians easily seduced by the rebels' revolutionary message. Far to the contrary they were clearly more sympathetic to the British. Regardless of whether they were written by common soldiers, noblemen, or bourgeois officers, the large body of diaries and corre- spondence that survived the war make it clear that the Hessians never sympathized with the constitutional arguments presented by the Amer- icans. Coming as they did from a country where even the average no- bleman was poorer than most colonists, they were also appalled by the Americans' unwillingness to pay taxes for British protection against the common French enemy. Most of all they were deeply critical of the hypocrisy of men who could proclaim "freedom" from oppression, yet persecute Tory loyalists and enslave and brutally mistreat a half million African blacks.

The Hessian treatment of blacks demonstrated, in fact, that the two societies were worlds apart in their definition of equality. So appalled were they at the denial of human dignity to slaves that individual Hessian officers tried to purchase or otherwise free blacks from their masters.

^{Hessian deserters and missing, 1776-84. These tabulations differ slightly from Atwood's, which total 3,014 (cf. The Hessians, p. 256). Source: StAMg, 411)4294: "Auszug von dem Abgang... des hessischen Corps..." [n.d.]}

The 2,949 Hessian missing and confirmed deserters listed by the military authorities did not run away at the first opportunity, or on the eve of the first battles. Nor did they desert following exposure to American propaganda and bribes. Even Hessian prisoners of war gen- erally resisted American inducements, choosing to remain interned rather than become settlers or enlist in the Continental Army. Rather, the great bulk of them fought on for several campaigns, choosing to stay in America only as the war was coming to a close and after they had already been demoralized by capture, idleness in garrison duty, or impending defeat.134 It was only at this point that the soldiers began to turn their attention to the future and to the impending choice between returning home and staying in the New World.

_{–– Charles W. Ingrao: "The Hessian Mercenary State: Ideas, Institutions, and Reform under Frederick II, 1760-1785", Cambridge University Press: Cambridge, New York, 2003.}

D. H. Fischer's 2004 Washington's Crossing has a chapter on the Hessians in the American Revolutionary War. On p.52 is a map showing areas of Germany from which German troops in the war came from. More than half (18,970 out of about 30,000) are shown as coming from Hesse-Cassel.

"Hessians" were German soldiers serving the Prince of Hesse, (or other German states) that had been recruited for "domestic" service, and then sold by their Prince to the British for service abroad. The ones from Hesse made up the single largest contingent of the these German mercenaries, hence the name Hessian but about half of the Germans came from other states.

These "sales" took place in whole units; that is the soldiers were sold "by the regiment" and served in the same units under the same (immediate) officers while in British service as at home. The British paid the prince,not the individual soldiers for this service. It's not like they were signed up individually by say, British agents, to be commanded by British officers.

They were not true "mercenaries" in the sense of putting themselves up for hire. The only one that was "mercenary" in this transaction was their prince. Their original intention/understanding had been to defend their homeland.