Federal law (ECPA) does indeed bar us from listening to stuff that is
neither telephone calls nor encrypted.  It also covers, among other
things, station to transmitter links in the broadcast service.  That
means that it is not legal to listen to the remote sites such as
temporary facilities at fairs, merchant premises, traffic reporters in
helicopters, et cetera.  By the way, listening to the traffic reporters
sometimes gives up to a ten minute edge in knowing where the problems
are.  The studio calls the information up to the traffic reporter in the
sky, and up to ten minutes later, the helo reporter broadcasts it to the
listening public.
The ECPA also prohibits unauthorized listening to SCA transmissions on
FM stations.  These 67khz and 92khz subcarriers often carry music and
other services for pay.  
You'd have to read the entire ECPA to find the full extent of the law.
Duane 
KA7JEX
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