The amount of "targeting" calls (nowadays about half of the time) that get called when the receiver ducks down to lower his pad level and the defender strikes the lowered head of the receiver makes this whole rule a joke.
You may as well hand games to offenses when you are asking a defender to do the impossible. If the only surface area of a receiver to hit is a helmet because they themselves are falling to the ground or bracing for impact, how do you hit him? And by the way, you have a millisecond to pull back your momentum because an offensive player made their helmet where their chest and waist once was.
There are egregious times where someone will pretty clearly strike someone in the head on purpose. But the amount of "Bang Bang" plays where the defense loses a key player for an inadvertant and questionable "targeting" call makes the game harder and harder to watch. It becomes selective foul calling.