After making a blanket prohibition of intercepting all electronic (i.e., radio) transmissions, the [ECPA of 1986] statute lists the exceptions. The first exception is that it is legal to listen to all radio transmissions which are "readily accessible to the general public."
This term used to be defined in the statute to mean radio signals which are: (1) not encrypted, scrambled, carried on a subcarrier or other signal subsidiary to a radio transmission (2) not transmitted over a common carrier communications system (such as the phone company) (3) not special transmissions such as point-to-point private relay transmissions for the broadcast services, not meant for reception by the general public. ...
At this point the only legal listening outside the broadcast bands is:
(a) a communication relating to ships, aircraft, vehicles or persons in distress;
(b) a broadcast by any governmental, law enforcement, civil defense, private land mobile or public safety communications system, including police and fire;
(c) transmissions on the amateur bands, citizens band or general mobile radio services as well as any marine or aeronautical communications system;
(d) satellite transmissions of cable programming as long as the transmission is not encrypted, there is no monetary gain by the viewer, and there is no marketing system available (meaning no one is selling the rights to view the programming via satellite).
(e) a radio transmission which is causing interference with any lawfully operating station (including ham radio operators), or is causing interference with any consumer electronic equipment, to the extent necessary to identify the source of the interference.