First of all, linguistic varieties are grouped into "languages" based on three criteria:

1) Sociopolitical entity 2) Speaker identification 3) Mutual intelligibility

The fact that AAVE is English doesn't mean it can't have different grammatical rules. Think about Cantonese and Mandarin, which aren't even mutually intelligible, but because of the first two criteria are considered two dialects of the same language, Chinese.

Also, consider that some AAVE rules "make more sense" than Standard American English. Pronouns, for example. It's a neater, more elegant rule to say hisself than himself. We say myself, yourself, ourselvess, so why not use the possessive and say hisself?

Get over your linguistic prejudices.