The point is that, in general, the behavior that the police engages in is not subject to the law. It is technically illegal, after all, to shoot a bunch of black people for no reason. The police do it anyway in contravention of the law because they know that they will be protected and believed.
The authority of the law derives from our joint agreement as a society to forbid certain behaviors, to encourage others, and to authorize the use of force on our behalf to protect this agreement.
The authority of the police derives from their authorization to use force and our assumption that that force will be deployed appropriately.
The two are really not the same thing! I assume that the majority of people who are against police violence are in support of the law. They just want it appropriately enforced.