He didn't block the plate. Cousins could have very easily slid directly over the plate without touching Posey at all. If you go back and watch the video again you will see Posey is on his knees outside the batters boxes.(pause it at the 12 second mark) Posey looked like he was going to go for a sweep tag. Was the play legal, sure. Was it clean, I guess. There is little doubt in my mind that Posey didn't want to and didn't try to block the plate.