There actually is a good reason, although it is becoming more outdated with each passing decade. The reason is that Germany and Japan are healthy and wealthy allies to the US now and so must be defended. But, given their recent history, the notion of each of them rearming to the point of being able to defend themselves totally freaks out the couple of dozen neighboring countries that suffered from them during WWII. If not for the stabilizing presence of the US troops, both countries would have been forced to develop nuclear weapons to defend themselves from Russia and China. The very notion of nuclear-armed Germans and Japanese freaks out all of the formerly occupied countries as well, especially China and Russia. We shouldn't have to station troops there forever . . . but there are valid reasons for having some of them there for the time being. The Russians and the Chinese may dislike us, but they trust us more than they trust the Germans or the Japanese.
