Actually - maybe not. There are departments that want the officers to stick with semi auto, above 26" weapons. As I mentioned above, there is someone here that discussed the decision by his dept, and the reasoning why the superiors chose the PS90 over the P90.
I think one reason was that they didn't want non swat members walking around with full auto weapons. Remember, not all cops practice with their weapons a lot.
Also, leaving a full auto P90 machine gun hanging on the rack in a police car might not be a good idea. Cops leave their cars running and unlocked all the time.
I've herad of depts giving out CX4 Beretta carbines for use in squad cars. The PS90 wouldn't be bad. it's shorter than an AR, and would be useful in an urban setting.
Sure, I'll bet the cops WISHED they could get real P90s. But, I can't say that arming every patrol guy in town with a full auto P90 would be a good idea.