Police don't really "protect" anyone, their job is to cleanup the mess and investigate after the fact.
If the police fail to respond to calls in the ghetto then the crime in the ghetto increases, for instance.
A big part of it was how the LAPD would only enforce laws in certain neighborhoods. The gangs were literally trained by the police that if they commit crimes on one side of a particular street, the laws will be enforced, but if they commit crimes on the ghetto side of the street, they will not be.
Sure they do, they discourage crime by finding and jailing criminals. Prevention is protection.