So you're saying Silva hit a grounded opponent in the head with a leg strike?
Legal is a referee's discretion. I've seen Big John blatantly ignore cup shots because the intended attack was shin to thigh and he regarded the foot following through to the cup as insignificant and incidental. Doesn't make groin shots legal. Same applies here. Yves saw the knee go into the chest as intended and ignored the accidental foul, which is 100% a foul, but ignored it, as he judged that the majority of the effect the strike had on Chael was from legal contact and basically overruled the foul. In essence, no one should argue that Chael would have been ok if it didn't hit his face, that's obviously bull shit. But that is still a foul.
It's like when any fighter (Koscheck) gets grazed and starts milking it. Sure, they're full of crap, but that doesn't mean a foul wasn't committed.
Anderson had that fight won. He just had an insignificant foul doing it.