http://cla.calpoly.edu/~jrubba/Ebonics.html
If all languages and dialects are linguistically equal, why do our schools and other institutions insist that only one kind of English is 'proper' or 'correct'?
This is the result of social prejudice, the favoring or disfavoring of particular social groups. It is no accident that the varieties of a language that come to be thought of as 'proper' or 'correct' are invariably associated with the upper classes, those who wield political, economic, and social power. These classes also control the educational and publishing worlds. Whatever kind of English, French, Spanish, or German they spoke became the standard dialect. Since these classes established the educational system and require mastery of the standard dialect in order to grant access to higher learning and powerful employment or social positions, they effectively impose their dialect on anyone who aspires to education or upward mobility. They also control what gets published. It is also no accident that the varieties of language that come to be called 'substandard' are invariably associated with groups that are the targets of prejudice: inner-city poor or working-class folk (Cockney English; African American English); rural peoples (Appalachian and 'cowboy' English); ethnic minorities (Native American English, 'Spanglish', African American English). The dislike of the groups was transferred to the forms of language used by them; the dislike of those language forms then became institutionalized with the establishment of an educational system, and continues to be promulgated and enforced today.
So the notion of 'good vs. bad English' is simply one more institutionalized prejudice. The belief that some varieties of English are inherently inferior to others is no different from the belief that some skin colors are inherently inferior to others: both are beliefs that rest on historical accidents of who had privilege and power at a crucial moment in history, not on scientifically-describable superiorities of one over the other.
Over time, attitudes towards dialects have been divorced from the groups that use them, so that many people now categorize a person as undesirable from the cue of how that person speaks: a black student who speaks 'well' (read: standard English) would no doubt be judged more intelligent, cooperative, and competent than that same student speaking 'Ebonics' (read: 'bad English'). Such results have been obtained in actual studies of language and dialect attitudes in other parts of the world.