Under capitalism the main police function is to break strikes and to repress other forms of protest against the policies of the ruling class. Any civic usefulness other forms of police activity may have, like controlling traffic and summoning ambulances, is strictly incidental to the primary repressive function. Personal inclinations of individual cops do not alter this basic role of the police. All must comply with ruling-class dictates. As a result, police repression becomes one of the most naked forms through which capitalism subordinates human rights to the demands of private property. If the cops sometimes falter in their antisocial tasks, it is simply because they – like the guns they use – are subject to rust when not engaged in the deadly function for which they are primarily trained.
No police organization is exactly the same day in and day out. Two essential factors determine its character at a given moment: the social climate in which the cops have been operating and the turnover of personnel within the force. An unseasoned cop may tend to be somewhat considerate of others in the performance of duty, especially while class relations are relatively peaceful. Even in such calm times, however, the necessary accommodation must be made to capitalist demands, including readiness to shoot anyone who tampers with private property. Otherwise the aspiring cop, if he is not kicked out of the force, will have little chance of rising beyond a beat in the sticks. By gradually weeding out misfits along these general lines, a police department can keep itself abreast of requirements during a more or less stable period in class relations.

– Farrell Dobbs, Teamster Rebellion

When do the police ever pass on a chance to hurt protestors?

That’s like their crack. Well that, and anabolic steroids.

For the same reason they will comply with such request from any private party? Try to pitch a couple of tents on your neighbor’s front lawn with your friends to see if the police doesn’t get you moving when they get called.