Are hard hits against the rules?

[Dave Gray]

http://sports.yahoo.com/nfl/blog/shutdown_corner/post/Broncos-rookie-safety-Rahim-Moore-defends-devast?urn=nfl-wp5513

Look at the first clip.  It shows a player getting laid out, but right after a ball bounces of his hands in what could've been a catch.  The defender hits the guy really hard, but it's within a reasonable hit window.  The ball is right  there...no time to react and judge that it was dropped.  The defender hits with the shoulder, and not into the opposing players' head.

Why the flag?

Was it simply a bad call?  Or is the NFL trying to regulate hard hitting, even if it pertains to making a play.

[Pappy13]

You can't lead with your shoulder anymore either. The NFL is really trying to get away from using a "hit" to break up a pass. Either you swat the ball away or you tackle the guy, hitting a receiver trying to make a catch to try to dislodge the ball is being discouraged. Once the receiver catches the ball and starts to run, then it's a different story.

So yes hard hits are against the rules, if it's while a receiver is trying to make a catch. The receiver is unable to defend himself against such hits.