this discussion has been here before. Moving policecars are just too unpredictable to display them on the map. By the time a user has committed the report and the clients have received the warning, it might already be totally obsolete. Over here "moving" could be anything from 5 to 150 mph.
Especially in areas with many turns waze would have to calculate a bubble which increases every second. (Maybe we could get the cops to use waze, too?
And I don't really see the point in knowing where the police might be driving around. They can only take your speed when driving right behind you (unless you're totally overdoing it). So your rear mirror would probably still be more reliable then the "attention, there could be a policar somewhere in a 10-miles-radius (growing by 1 mile per minute) around your current position" warning.
Just "drive safe" and respect speed limits. Still the best way to keep you out of troube...