How was this UNC-South Carolina hit not a penalty, under current rules?

South Carolina cornerback Victor Hampton brought back a punt that he probably should've fair-caught in the second quarter against North Carolina. He lurched forward and was struck in the back of the head by Brandon Ellerbe after he'd already hit the ground.

Considering the current focus on avoiding head injury, how was this not a penalty? The talk of the offseason, as far as on-field rules go, was the new emphasis on helmet-targeting hits. The rule was modified to include ejections for players found guilty of needlessly striking another player in the head. That sure looks like what's happening here.

We assumed the rule's first controversy would be an ejected player, not a hit that looks like it should've been flagged. (That flag in the GIF, by the way, was for something else.)

ESPN reported after the hit that Hampton suffered a neck sprain. He was spotted with a towel over his head on the sideline.

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