What would happen if the police were politicized and what are the principalities and powers so afraid of? It's the same fear that means the military have been traditiaonally banned from unionising.
If you get the sharp end of state-violence discussing their views and values, forming into groups, thinking about their rights and the rightness of their actions. If you had a grown-up autonomous police force doing some serious introspection then how could you maintain it's present role as living out someone else's agenda? You just couldn't.
"We need to be on the beat more?"
"It's what the voters want."
"It's not what we want we get hurt and abused"
"Why do we get hurt and abused."
"Because of all the fascists."
"What's a fascist?"
"Someone who subsumes there identity into a group in order to gather enough force to impose their morality on others and .... oh,.... I see."