Because Horry was leaning so much, it was called a foul. If it's a foul, then it's illegal. If it's illegal, then why does Horry, in that situation, do that?
Any pick that is set like that is going to be called a foul. He's leaning so far into West that he's standing on his toes. There's a difference between bracing oneself to absorb impact and leaning your weight into a guy to maximize impact.
I think Horry leaned too far, and knew what he was doing, but West jumping just exacerbated the hit.
Here's a good hard pick:
http://www.youtube.com/watch?v=RkK-DhdpLuo
Gray stands straight up.
We have seen Brad Miller set hard picks, yet he doesn't have to lean like that, even when he braces for impact. Horry is 6'10'' 240lb, so it's not like he needed to do that to keep his footing. He could have very well stood straight up and flopped like he does half the time.