Now this one on Davis was an actual Helmet to helmet hit that was missed by the refs. In fact what the defender did there is the epitome of the rule. A fine should have been levied. He launched himself with the crown of his helmet.
The gif is right in front of you, yet you still say it is illegal? The helmets never touch and Chancellor puts his shoulder pad into the chest of Davis. The only thing illegal is hitting a defenseless player.