Because of prevailing attitudes and upbringing, a statistically significant portion of those cops come from the same pool that generates the white nationalists.
Those cops rise in the ranks to be the people in charge of the police force, where they have command over which protests get dealt with harshly and which don't.
Because of the perceived nature of a cops authority simply because they carry a badge and a gun, the people who most want to become cops are the very people that should psychologically never be allowed to become one.