France has "federalized" police in most of the country and they don't have this problem. I'm with you, that doesn't seem to be the causal factor.

I don't have great insight into police, but I've had a little into a few county governments and school districts, and the impression I've gotten is the opposite: the more local interested are insulated from state or federal oversight (de facto, if not de jure) the more wildly-corrupt and tyrannical they get.