When is a penalty not a penalty?

I'm not being a wiseass, this is an honest question that I don't know the answer to: Why wasn't Peterson called for a personal foul when he ran over Gay? From what I could see, AD clearly leads with his helmet and hits Gay in the face, you can see Peterson lower his head so that the top of his helmet strikes Gay's facemask. That hit would've leveled almost anyone and could have broken a jaw. Is this not illegal? Defensive players will get penalized ten times out of ten for leading with their helmet and striking the helmet of the QB. Not just QBs, but RBs, WRs, and any other offensive player, for that matter. So I would think this one should've been a penalty on Peterson, unless I'm misunderstanding the rules or something. Anybody here know the answer to this one? (Plus, it oughtta help quiet down the MIN fans that keep complaning about the tripping call).

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