Friedrich Engel and Josef Scheungraber were investigated and first convicted in Italy for killing Italians. Engel was not imprisoned because of his age. Heinrich Boere was a Dutch citizen, convicted in Holland of killing Dutchmen, who fled to Germany after the war.
All these men were convicted of direct involvment in murder. Can you name a German-born, German citizen imprisoned for war crimes like Demjanjuk for being drafted as a camp guard?
So basically you're going from a position of "No Germans have been prosecuted" to a position of "Okay, Germans have been prosecuted, but they were murderers. And volunteers."