Whenever a player, especially a star player such as Buster Posey, gets injured, emotions will get heated and controversy will ensue. Here are my thought on the matter.
I have refereed probably over 350 soccer games in my life. My primary job is to protect the players' safety and make sure to protect the integrity of the game. This is no different than any other sports official. In looking at a play like this, you have to look at two things:
1) Did Scott Cousins break any rules? Simply put, the answer is "no".
2) Did Scott Cousins take any liberties with Buster Posey? Absolutely not. He went towards the outside of the plate, and hit Posey on the left shoulder after he went to block the plate. The injury was to the left ankle because it was bent backwards. He was playing the game by the rule. If he was taking liberties, he wouldn't have checked to see if he was OK.
Now the other question is, should the rule be changed, to protect the players safety? Personally, I think so. The reason, you have a catcher who is stationary, that this going up against a guy who has a 30 yard head of steam. He is a sitting duck, and that is just not a fair situation. I also don't think that it adds anything to the game, unless you enjoy snuff films or kicking puppies. Change the rule so that:
1) You have to slide into the plate, and
2) The catcher can only block the plate IF they have posession of the ball.
This will make it fair!