Generally speaking, there are certain restrictions even in jurisdictions that allow the use of scanners and radar devices. In particular, scanners can occasionally pick up signals such as private cell phones, creating situations that breach privacy laws. According to most restrictions, it is illegal to own, operate, or sell scanners that are capable of listening in on cell phones or wireless home phones. As breaching cell phone privacy is considered a federal law, the act itself is considered illegal across the United States.
For many jurisdictions, it is also illegal to break into encrypted channels to listen in on information that is not supposed to be available to the public. This means that those listening in for police information in order to help them achieve a criminal end, such as speeding, are often breaking multiple laws.