Yes, they assume the risk when they enter the game and yes, they are paid millions of dollars to do so. They are ultimately paid that kind of money because, it entertains you and I. Would we really be less entertained if MLB implimented a simple rule that might reduce some of the risk and possibly sustain a star like Buster Posey's career a little longer? For our entertainment.
Posey didn't have control of the ball and it appears to me that there was a path the runner could have taken directly to the plate and avoided some if not all contact with Posey. In this situation, IMO, a rule needs to implimented. That's all I am saying. Don't change the game entirely but take away the blindsided hits and discourage players from simply trying to take out the catcher. He didn't go after the ball, he just lowered his shoulder and went after Posey.
Sorry, I might be too sissyfied for you. I'll go put my slippers on and grab a cup of milk before bed time. Nighty Night.