One thing to consider is that the US occupation was an occupation and didn't become a guerilla war. The devastation of the war and the fact of their defeat had discredited regimes based to a large degree on national and military glory, and the allied provision of generous economic assistance and food aid, and support in Japan for land reform, went a large way towards winning over the populace.

This isn't to say that the occupations were spotless, or that no atrocities occurred during the occupation period. There were certainly more than a few rapes, and US troops in Japan took advantage of prostitution that was motivated in large part by desperate poverty. (I'm more familiar with Japan, so this post speaks primarily to it; the German situation was broadly similar in terms of defeated enemies and foreign policy motives, but I'm sure there are differences in the details I'm overlooking) The governments the US supported were certainly more democratic than those which preceded them, but keeping the communists out of power was also a US goal and censorship in Japan was often turned towards that end.

But the war was over, and the social dynamic of insurgent warfare where civilian and enemy soldier couldn't always be distinguished - a dynamic that leads to continued hostility and often to massacres - simply never developed.