I've lived in a few areas, and the 'black part of town' is always some rundown ghetto compared to the rest. They talk differently, and can talk back more aggressively than the others. It's the only race that seems have its own unintegrated subculture: Someone who's Asian, Jewish, or Mexican after 1 or 2 generations might have distinct friends in a friend graph, but if you meet them on the street they're just another person. Even visiting foreigners stand out less than black people in the United States.
If I were a police officer interested in fining people, I'd head to the rundown parts of the city where the "broken window theory" says more crimes will be committed. And there seems to be more black people there. So I'd probably end up fining more black people than not.
My guess is that whatever gives black people a 'black accent' or voice is also what's responsible for the fining and bad subculture. That is, a black person who cannot be easily distinguished over the telephone would be statistically similar to the population as a whole. In particular, they wouldn't be fined as much.
I would also guess that black children adopted by middle-class white families don't have this problem.