Cops dont like it because they dont understand the "cop cars are cool" thought process. To them theres three kinds of folks who drive police looking cars.
1. Police officers
2. Poor folks in the ghetto who buy old crown vics and slap 24s on them
3. "Wackers", folks who impersonate officers for nefarious reasons, also just folks with an unhealty fascination with law enforcement, sort of super wannabees.
They think you fit into category three, and they dont like folks like that. That creeps them out, and the last thing you want is a whole bunch of guys with guns creeeped out by you