It was uncalled for even if others did it and didn't get caught. I'd have no issue if he was fighting face to face unless he started it for no reason but he didn't fight face to face in the punches he threw.
I'm sure he was amped up and trying to retaliate for the unfair punch he got when the BYU coaches were holding him but either way he shouldn't have thrown the "cheap shot". That's a punk YouTube move.