Umpires need to be better at working out what a player's intent is. Walters is usually just trying to get low and under the tackler, and they don't have to be careful about the tackle because they know the umpires have marked his card as Aboriginal, so he's staging for free kicks.
Selwood's parents give money to orphan pupppies, also he's white, so they ignore all the times he clearly sticks his head into the path of the oncoming player.
It's very obvious but they can't see past their prejudices. Whatever those may be.