Because idiots like Giants GM Brian Sabean and Posey's agent Jeff Berry are whining about the play like elementary school girls. Both have heavy financial interest in Posey being healthy, so they feel they are being slated by Cousins themselves. They are having trouble accepting that injuries do happen from clean plays. Both of them need to STFU. And this is coming from a Giants fan.
I don't know about you but in the Bay Area it's all over sports and it happened a week ago. Last year Carlos Santana got a season ending injury on a play at the plate, Ryan Doumit, (Pirates) got injured at the plate a few days ago as well. It's because Giant fans suck and whine and complain all the time, you cant prevent it, it just happens, no rule could improve the situation in sports. Scott Cousins feels terrible and wrote to Sabean and Posey and those @ss holes basically said screw him and didn't write back. Most players who were there immediately said it was a clean play when asked. Giant fans need to suck it up, this happens to John Buck and Aaron Rowan injured him, no one is talking about it and it isn't a big deal.
You are right, if "Joe Schlabotnik", of the Nationals got wiped out on a play at the plate on a rainy Tuesday, nobody would care very much.
But the fact is, last years Rookie of the Year, and winning world series catcher, was knocked out for the season on a possibly avoidable incident.
It's good to talk about these things... the game evolves. If nothing ever changed in baseball, no one would use gloves, or helmets, and the batter could tell the pitcher what pitch he wanted.
I don't know if we need a rule change or not, but baseball should take a long hard look at plays like this, before someone is paralyzed by devastating neck or spine damage, or suffers some other career ending injury.