I never made this assumption, to do so would be putting words in my mouth. I have a Master's Degree in Speech Pathology (and a Bachelor's in Linguistics). Part of the curriculum besides Voice Disorders, is to study Speech Science, in which we do look at vocal patterns, pitch, intonation, inflections, etc. There are patterns that can be identified for certain people, and it may go back more than one generation. For example, my mother was raised in the Midwest and said the word "roof" differently from most people in California, where I was born, yet being around my mother, I picked up her pronunciation and would sometimes say [roof], like book-short u, instead of [ruf]- long u. People would ask where I was from because of that.
Gang wannabes, although they may adopt the lingo, don't usually also pay close enough attention to also mimic the pitch and other intonation changes. There is an actual identified speech pattern called "Black English". It has certain linguistic characteristics to it, such as reversing the skin the word ask to become [aks], and simplifying the verb "to be", such as saying "I be black" instead of "I am black".
Hope this helps answer some of your questions...
I don't care what your master degree is in!
Ebonics isn't spoken by all blacks and to imply it is, is ridiculous!!!