The players are not being required to leave the game because they may have suffered a head injury. they are being required to leave as part of the punishment for not having the helmet on properly. If we determine the foul was why the helmet popped off then they are excused from that punishment. If not, then they are not excused. So, as was said, this leaves the very real possibility that after a big hit, the hitter will be penalized AND the hittee will be leaving the game for a play. Lots of coaches, fans and players are going to struggle with that one.
Coach: He roughed my QB and because of that my QB hit the ground and his helmet came off. He should not have to sit.
Zebra: Yes coach, your QB was hit a tad late so we flagged the hit. But even if the hit had been a tad earlier and been legal, your QB still would have hit theground that way and his helmet would have still come off.