In another thread that was talking tangentially about the soy/nice guy, I think I realized the same thing. I came up with the term "Cultural Quisling" to describe it.
They'll ape whatever mannerisms they have to and say whatever they think needs to be said to curry favor with the "in" group, for personal benefit, and then try to deny ever having liked all those things when the tides of power change, ignorant that everyone darn well SAW him shake the Nazi's hands when they marched in, and aren't going to forget it.
Nice guys have no loyalty to anything but their own desires, and will sell out anyone and anything in a heartbeat if it looks like there's a pussy angle in it for them, and then whine whine whine when called out on it, belatedly realizing they backed the wrong horse.