its a different dialect of english. When comparing with the mainstream so-called proper English, then it may perceived as grammatically incorrect, but it is a valid form of communication. We all know what they mean. Usually this tongue is conditioned and therefore makes it harder to undo in everyday discourse. Language is a quite dynamic facet of society contrary to what many people here may think. New words and phrases are introduced all the time and become commonplace. Does this mean they are necessarily wrong ? If we all conformed to the status quo nothing would ever change. Free your minds people.
Also, why would they do it cause they think its cool? you could argue suburban kids (white, asian, and even BLACK) kids do it because they think they're cool because they heard it in a 50cent song or something. But, if you grow up around people who use this language, you will become accustomed to it and therefore communicate effectively within the group.
The original poster is an idiot for saying "black" people. Stereotyping to say the least, one of my biggest pet peeves. This really urks me when people say things like, "Why you actin' all white?" or "Why you tryin' to act black?". Why are people from a particular continent, Asia lumped into one group and you automatically think Chinese or something? In fact, part of Russia is in Asia. Who comes up with this crap? Its not right or politically correct to do so!
Back to my original point, there are blacks who live in the suburbs who can't relate to what black rappers talk about as well as there are whites in the city who can. I suggest you read some books on racism.