How is this Dangerous Helmet Hit on Amari Cooper Not a Penalty?

Amari Cooper took a brutal hit to the head from Brad McDougald in the second quarter of the Raiders game vs. the Seahawks. Shockingly, there was no flag thrown:

As ProFootballTalk pointed out, the only real question on this play was if it was an illegal hit on a defenseless receiver or a lowering the helmet penalty. Somehow the officials missed it.

Earlier in the day, it was reported the Raiders are shopping the former first-round wide receiver.

Cooper went to the locker room following the hit.