> Which people? The Europeans were occupied or liberated under effectively caretaker governments.

They were occupied but they weren't entirely busy: while "low" people were happy to kill ex-Nazi collaborators themselves, it's the post-war governments (all of them, USA's included) who needed, with those trials, to manifest a re-establishment of the rule of law once again. 80 years later we can see it's been a hypocrite farce in every part of it, but it saved lives, those that were worth of living, although spared Nazis, fascists and sometimes communists too.