I mean, the Fed may be juicing the futures at the "right time" (IE panic/support areas), but I don't think they are buying the cash markets.
I just think US investors are some very arrogant players in the world market place. They've been bailed out so many times when bubbles burst, and the bubbles are always allowed to reach such comical levels that "investors" get disconnected from reality. They live under the illusion that the king dollar cannot be unseated, the CBs are omnipotent/infallible, that worldwide market crashes are unlikely to affect the US and that valuations don't matter.