Product of their environment....
Lets stop pretending like its only white people who do this, I know many black people who do it as well.
1. Some do it cause they think its cool (both black and white)
2. Some do it because its how/where they were raised and have no idea they are doing it (both black and white)
The ones in #1 annoy me (both black and white) the ones in #2 are more tolerable but there is a time and place for everything, talking with friends you are more comfortable and you are more likely to speak how you normally speak, but if you are in an office you tend to filter a lot more.