Living in the Ghetto

The movie Boyz n the Hood shows the different types of black youth that live in the ghetto. Ricky is the good kid going to college to play football. His brother Doughboy is the kid that fell prey to the gangster life and Tre is the kid that is friends with both brothers. Throughout the movie Tre does not take sides of either brother but at one point of the movie Tre is pulled over by the cops. During this traffic stop Tre is harassed by the cops and is verbally assaulted basically for the fact that he is black. Why does society think that just because someone is black that they are a bad person or for that they are doing something wrong?