First off, it gets on my nerves too.
Having said that, language is fluid. It is made up of colloquialisms, regional dialects, social strata, etc... this no different. Many people who speak "proper english", and who feel that they are above such base regional/cultural dialects, are generally quite comfortable ending sentences with a preposition or beginning sentences with a conjunction. How many people here have had their knuckles rapped or their mouths washed out with soap for ending a sentence with a preposition? From what I gather, most teachers don't bother to even point out such flaws to students anymore because the rules have relaxed for common usage.
Want to put the ebonics into perspective? Go talk to someone with a cockney accent, or heck just visit Wales. These people have been "slaughtering" the english language since before the Americas were founded.