[OnyxCougar]

From Grants link, confirming Brianna's point about their choice to be different:

AAVE's departure from Southern American English was a natural consequence of cultural differences between blacks and whites. Sociologists, linguists and psychologists, however, believe divergent development of this kind is often passive subversion. Language becomes a means of self-differentiation that helps forge group identity, solidarity and pride. In the case of African Americans, AAVE has survived and thrived through the centuries also as a result of various degrees of isolation from Southern American English and Standard American English--through both self-segregation and marginalization from mainstream society.

edit: It occurs to me that this reflects the whole "African-American as a distinctive cultural subgroup" mindset as opposed to American folks who happen to be black.