It is
not illegal to monitor a trunked system anywhere in the United States, regardless of the hardware in use.
It is illegal to monitor any
encrypted traffic, trunked or not. It is also illegal to gain access to a
proprietary protocol by doing anything that would violate the intellectual property rights of the protocol owner.
Practically anything that can be done with an unmodified scanner, or with a commercial radio (except transmitting without license or authorization), is generally legal in the U.S.