I figure I should create this before the second game is played and this becomes entirely moot.
Yes, his helmet was coming off. But he didn't just take his helmet off, he tossed his helmet to the ground, and he celebrated his sack. If he had removed his helmet and held on to it, I think you could have made the case that no penalty was deserved, but the celebration plus tossing it to the ground got him the penalty (plus the refs probably didn't know it was coming off in the first place). Hopefully, the next time it happens, he will opt to not celebrate at all when the helmet comes off. Don't give the refs a reason to throw the flag.