Broadcasters first started making use of the "people's" airwaves in the 1920s, and it quickly became apparent that some rules needed to be established. For example, radio stations needed to reserve frequencies to prevent others from making use of them. As more people bought radios and different kinds of programming were developed, the Federal Communications Commission and other bodies paved the way for certain laws to be passed by Congress. Even though radio is no longer the most popular media around, there are still laws to govern use of the airwaves.
Licensing
In order to legally broadcast, radio stations must be licensed by the FCC. This involves purchasing a frequency or a station that already possesses it. When someone broadcasts without a license, even at low power, they are operating a "pirate" radio station."
Public Interest
AM (amplitude modulation) and FM (frequency modulation) radio operates in a small sliver of the electromagnetic spectrum. Much like the air or sunlight, there is a finite number of radio frequencies, and the U.S. government has maintained that they must be maintained in "the public interest." The FCC requires all stations to be licensed and that all stations maintain a file that documents what the station has done to operate in the public interest to help justify the station's possession of a license. According to the FCC, this file must be accessible to the public during the station's operating hours.
Emergency Information
In order to give the public the information they need to stay safe, radio stations are bound by law to broadcast emergency notices. For example, if there is a tornado watch in the area, all broadcast stations must report about it. In terms of radio, this means at least a verbal interruption of programming.
Payola
Many radio stations play music, and this exposure can mean a lot of money for the record companies who benefit from sales of the records. In hopes of getting records repeated airplay on a station, record companies would pay influential DJs to play certain records. In the 1950s, this was ruled illegal and was called payola. The law now states that a station must disclose if they receive money in exchange for presenting programming. That's why all infomercials on radio are preceded by such a notice.
Power Reduction
FM radio signals travel in a straight line, which means their range is restricted by the curvature of the earth. AM signals, however, can bounce off of the ionosphere, allowing a station to be heard up to hundreds of miles away from its broadcasting tower. The FCC mandates that most AM stations reduce their output at night, so as not to interfere with other AM stations.
