What Brian said.

Unless things have changed from 9/11 & homeland security, the law is/was that you could receive any signal that's being broadcasted and it's up to the sender to encrypt it to prevent you from receiving an intelligible signal.

Plus lets face it, it's not practical to enforce a law that prevented you from listening in. Lets face it, little kids with a science project could end up going to jail.

This is also why most police departments have both open and a secure radio nets. <img src="/forums/images/graemlins/kewl.gif" alt="" />