Renshaw DID endanger somebody - Farrar. Renshaw was DQed not for the headbutt on its own, but for following it almost immediately with a wild and reckless sweep to the left that nearly put Farrar into the crowd. Was it as mad as Ferrari's move today? Possibly not, but it was two separate incidents in close succession.
Think of it like football. Renshaw committed two yellow card offences within about ten seconds of each other, so received two yellows = a red.
Ferrari committed an offence that some people feel was worthy of a straight red, but others felt should only be a yellow. Hence why he's still there, but Renshaw was ejected.