If you are going to limit the ability of the defensive player to use the crown of his helmet you have to limit the ability of the offensive player as well. It puts the defensive player at a disadvantage from both safety and competitive perspectives. For example...
The defensive player is prohibited from coming in helmet first and creaming AP here. AP has no such limitations, however, so he lowers his head and lays the defensive player out. Watch the video at the 30 second mark, it's obvious that AP's helmet pops Gay's helmet. It's BS to help and protect one player (offensive) and hurt and endanger another (defensive).
If you hate this rule because of the safety aspect (it is somehow devaluing the game because it's not as "tough" as it used to be) than at least try to appreciate the fact that it levels the playing field competitively for both sides of the ball.
By the way, this was the first youtube result for a search of "adrian peterson truck", not something I had to spend a significant amount of time searching for to fit my point of view.