Who can't defend themself? The catcher/Posey? If you can't man up, don't block the plate. Make the tag off to the side.
I agree with Bobby Valentine's assessment of what happened. Don't start making new rules, just change the culture and the way the catcher blocks the plate.
They changed the rules in the NFL because they had to. Brady was lost for a full season because some big doofus took out his knee (intentional or not). Carson Palmer was never the same after that cheap shot in the playoffs. It's not "putting a dress" on the quarterback. It's protecting a multi-million dollar investment. I think they should do away with the slide rule, though. That's not fair to the defense.
As for baseball, you're right. The catcher shouldn't block the plate. I've felt that way forever. But the guy who got hurt left some of the plate open and the guy still took him out.