It could be something as basic as the fact that poorer people are perceived as easier to get away with doing stuff to, as it's not just certain races or ethnicities, but drug addicts and homeless, for example, from poor areas getting the same shitty treatement regardless of skin tone.
Murderers, rapists, Mafia, the richer, higher profile criminals, practically get the red carpet treatment in comparison when arrested. They're usually treated better, processed quicker and more carefully, etc, because the system doesn't want to risk blowing the case and having them set free, and because they don't want to chance getting sued either.
A homeless druggie, in comparison, gets "resisting arrest" tacked on to their charges to cover for the bruises acquired during arrest, and their court appointed public defendent who gets paid by the same system that pays the cops and judges gets them to agree to a plea deal so they can get home sooner and that is that till the next time because they have no other options.