I'm curious, which company is it? PM me if you don't want it public.
there are two reasons this is done. The first is it burns fuel, and fuel is expensive (or so my supervisor keeps telling me), and leaving the trucks running while at post is a waste of money. the second is that in NJ (like in Cali) it is illegal to idle a diesel engine. you see the warning signs in the parking lots of Wawa all the time saying not to idle your engines.
It def sucks, and most places don't enforce the laws about emergency vehicle idling. instead of asking people on a web forums, why not call the nj department of labor and ask them?