The NFL says that James Harrison’s $75,000 fine was for his hit against Mohamed Massaquoi. His hit against Josh Cribbs, which also knocked its target out of the game, was legal. Here's the NFL's rule:
h) If a receiver has completed a catch and has not had time to protect himself, a defensive player is prohibited from launching (springing forward and upward) into him in a way that causes the defensive player's helmet, facemask, shoulder, or forearm to forcibly strike the receiver's head or neck area -- even if the initial contact of the defender's helmet, facemask, shoulder, or forearm is lower than the receiver's neck.
You can compare the two hits here: