It is illegal to use anything you eavedropped on for a business 
or for an illegal use.  Jurisdictions that ban radar detectors or
police monitors, are essentially making the claim that there is no
other reason for a radar detector or police radio monitor. 

The results of fighting these claims in courts have been mixed.
The federal courts are not anxious to intervene and state courts
have sometimes held that the feds have exclusive jurisdiction
and sometimes they have not.  A lot of state courts do not have
enough imagination to see any use for a radar detector besides
avoiding law enforcement action for speeding.

When you buy a radar detector, amongst the guarrantees, instructions, 
explanations, and other detritus, is an invitation to join an
association dedicated to preserving your right to use a radar detector.
(I promise I am not making this up!)

I do not think radar detector manufacturers would be making any 
headway at all in courtrooms if police departments enforced speed 
laws with a strict eye to public safety and a blind eye to the fund
raiser aspect of tickets.  