Often times certain frequencies are illegal for hobbyists to use because they may pose a security threat, claims About.com. These banned frequencies are often used by government or law enforcement officials, and information obtained from these frequencies may be dangerous in the wrong hands.
Some government and law enforcement agencies prohibit the use of certain scanner frequencies as a precaution to help protect the public. There have been documented instances in which criminals utilized scanners to listen in on frequencies used by law enforcement agencies in order to commit crimes or avoid getting caught, explains About.com. For this reason, different state or local jurisdictions have implemented laws to ban hobbyists from using certain scanner frequencies. In some areas, all frequencies are accessible and legal with the proper hobbyist permits. In others areas, some or even all scanner frequencies are prohibited for public use.
It is also worth mentioning that even in jurisdictions where a frequency is legal, utilizing public scanner frequencies in order to commit a crime may still be grounds for prosecution for a crime such as "obstruction of justice." Since these laws vary widely, it is always important to check into the legality of particular frequencies before utilizing them.