It is not illegal to monitor any public service frequency anywhere in the USA because the US FEDERAL government says so, in codified federal regulations.
You CANNOT however disclose the contents of same, nor can you rebroadcast to others.
Some states do indeed try to confiscate scanners and/or amateur radio equipment installed in automobiles, however, in each and every one of these cases, where contested in Federal court, the Federal regulations have prevailed, and the equipment needed to be returned to the owner.
Excepted are cases where illegal activity of a criminal nature occured.
So, if you have the bucks required, you indeed can win and retain your equipment, and possibly litigate for just compensation.
OTOH, if you want to avoid problems altogether, just keep it out of sight.