Why don't they just do magnetic mount mini bars instead of the hideous things they are using right now? They could even do a permanent mount mini bar on the special straps they have if they want it to be more secure, then they could still remove it easily. Also, they could always do slicktop packages on the vehicles and have a LED beacon they throw up on scene for identification purposes.
My guess is that it's extremely expensive for the police department to order PPVs, so they use cars that were already destined for Hawaii, rather than order cars that only go where ordered for police use.
We were in Hawaii a few months back, and the practice seems to be prevalent there of using your POV for police use. I saw blue lights -- mostly strobe, a few had minibars -- on all sorts of cars, from Subaru SUVs to Honda Accords. They tended to be at what I'd consider overtime gigs: parked along roadside construction, a few at beaches in the middle of nowhere/land's end, and specifically in Hawaii county, in the numerous little towns far from the few established cities. There, I don't think I saw any marked patrol vehicles outside of Hilo during our entire visit, minus the few national park coppers in their pickups.