Here in California, there's an
obscure law with the common wording of (any person who)"...intercepts any public safety radio service communication, by use of a scanner...for the purpose of using that communication to assist in the commission of a criminal offense or to avoid or escape arrest,...or who divulges to any person he or she knows to be a suspect in the commission of any criminal offense, the existence, contents, substance... of that communication concerning the offense with the intent that the suspect may avoid or escape from arrest, ... guilty of a misdemeanor. "
During my 30+ years in the news media, I've never seen it really enforced. My company photo car had at least three scanners (each with its own antenna), plus a two-way radio and cell. I've encountered my share of wannabees that tried to tell me I couldn't listen to "their" frequency, etc., and legit cops (and a few FEDS) who didn't know ho easy it was to monitor their systems (oh, for those glorious days of conventional FM systems).
A little common sense and discretion goes a long way.