Having discussed with people in the comments, and having had many people including yourself raise good points, I think I've heard a litany of right answers.

It's probably some combination of racism, of support for the Jewish community/Israel, the fact that the U.S. had less direct involvement in places like China during WWII, guilt as you mention, and I think it's important to note that after the war, the U.S. installed a capitalist/democratic system in Japan and basically made them into an ally.

I think the U.S. has for a long time now seen Japan as useful, so maybe it didn't serve our purposes to have them vilified. With communist China right next door, it makes sense that the American ethos would favor Japan.