Why? You get angry at something and you lash out and include the word 'black' in your insult and suddenly you're a racist. It's a credit to the AFL that there isn't more of to be honest.
You can sledge by calling someone a weak or gutless so and so but if in the heat of the moment your chosen adjective is "black" then whether you consider yourself a racist or not it is clear that deep down, race is an issue.