Sure, there were no public trials to provide a catharsis for the crimes against humanity, still though, two atomic bombs on major cities vaporizing babies, men and women alike doesn't sound like 'mostly free'.
Sometimes you just have to send a very strongly worded message in the form of nuclear fallout in order to get the point across. I don't think the Japanese would have given up as easily if a not insignificant portion of their country wasn't turned to glass in an instant.