Travel a lot on US highways and very often experience marked and unmarked police cars speeding with no indication that they are responding to an emergency.
If they are required to speed in non-emergency scenarios, there should be some indication (such as amber flashing 'caution' lights) to alert other road users of the hazard they are causing.
With no visible indication, they are no different to any other road user and as such should obey the posted speed limits and be prosecuted the same way if they do not.