Originally Posted by klockWork
Even if it was illegal refs will never call a personal foul for hits against a running back on rushing plays. It's only call during the act of catching a pass when receiver can't protect themselves.
Helmet to helmet isn't illegal in the NFL unless it's against a defenseless player (not just receiver). Contrary to your comment, there's 8 specific categories of defenseless player listed in NFL rules.
- A player in the act of or just after throwing a pass;
- A receiver attempting to catch a pass; or who has completed a catch and has not had time to protect himself or has not clearly become a runner. If the receiver/runner is capable of avoiding or warding off the impending contact of an opponent, he is no longer a defenseless player;
- A runner already in the grasp of a tackler and whose forward progress has been stopped;
- A kickoff or punt returner attempting to field a kick in the air;
- A player on the ground at the end of a play;
- A kicker/punter during the kick or during the return;
- A quarterback at any time after a change of possession;
- A player who receives a "blindside" block when the blocker is moving toward his own end zone and approaches the opponent from behind or from the side.
None of those eight apply to this play, and therefore, it would be considered a legal play in the NFL.
Just to give an example, the following play was specifically ruled not to be illegal by the NFL last postseason.