I've never really understood this.
First, wouldn't you call them working class folks? I don't know how it could be said they're being exploited because I'm not sure what they're producing. Maybe you could say they're producing an orderly society.
Second, sure, some of police officer's work is definitely protecting bourgeois property interests, but to say this is all they do, or even most of what they do, seems like a stretch to me. Enforcing speeding laws are in the public interest, for instance, and not a bourgeois conspiracy. So, semantics aside, a constabulary force would be required Come The Revolution, unless you believe people's farts will smell of roses under socialism.
http://libcom.org/library/violence-against-police-commune
http://libcom.org/tags/police