Not so long ago, the police force didn't include their people. Many white protesters come with a different life experience and different point of view about the function of police. They may resist the hard work of understanding that their life experience is limited and their point of view may not be the only valid one. The point of view of these white protesters is also important. Plenty of evidence does support hard criticism of the functions of police forces.
Police have a very distinct role within an oppressive and violent structure.
They are given force of arms to protect private interests and are given the power to hurt and kill people. Although they are frightening and can seem all-powerful, police are really doing others' bidding. The police are workers. White protesters sometimes forget this and set up the cops to be the enemy. What this actually sets up is an opportunity for unconscious racism to show up in scary confrontations.