Well, you can start from the fact that language is a living, changing thing. The only real rules of the language are descriptions of how people are using the language. Even after they put rules to it, those rules have had to evolve as speakers change how they use English. It's not like we still use Shakespearean English as the standard of correctness anymore.

So, the set of rules that are written is just a description of how some people are using the language at the time. Can you take a guess which people's use these rules are based on? You can bet it isn't going to be the black people. And then these people can use these rules, which are just a description of how they use English, to say black people are wrong.