The Military base I currently work on has a Civilian police force that utilize large SUVs (Chevy Suburbans) and pickup trucks. These vehicles are always on and running even when officers are away from vehicles. Whether a few minutes or thirty minutes. This idling causes heat damage (wear/tear) to the engine and vehicle components as well as gasoline use. It seems we could save a lot of money by just turning them off.
I dont know the official reason why? Maybe to keep the air conditioning or heating on depending on weather is my best guess.