With Germany being partitioned into so many different zones and having the nation split in two by people who had a much closer relationship with Germany over centuries, I think they'd be a little more gung ho on trying to overhaul things (out of spite or revenge maybe) and things undoubtedly changed again when east and west reunited - lots of history between all the countries involved.

US and Japan, the relationship is much shorter and Japan was seen (before the war) as the most "civilized" Asian nation. Heck the US kind of didn't care when they were annexing Korea and going to war with Russia. US was a bit more self serving and didn't see any other country that could stand up to the communist bloc in Asia. In Europe, the UK and France could probably rebuild and be a good counter weight even if Germany didn't pan out 100%. Again, hard to pinpoint - interesting to think about why one country did a huge 180 and another just.. didn't lol.

I think it's less about military strength/defeat and more that the US wanted a strong East Asian ally in the face of China and Russia. Lots of institutions were allowed to continue and they didn't try to change things too much day to day for Japanese people. It paid off and Japan had crazy economic growth at the cost of keeping the good/bad parts of their culture intact.

Whether or not a bloodbath ensued before total surrender, I would wager the US was more scared of China/Russia at that point. But who knows, it's never one thing or another in history. Too many factors.