That should have been flagged. He clearly hit helmet first. Though I don't believe it was "dirty", it was clearly helmet to helmet. If that's how they define the rule, then it should have been an easy call. But it should have been a fumble also... Ironically, if he'd have lowered his head, it wouldn't have been helmet to helmet. So which one is worse? If that tackler keeps his head up, the chance of helmet to helmet contact is greatly increased...