That is a different matter since the airwaves are a scarce resource that can't be used by everyone at the same moment, therefore it has to be regulated somehow. As an example, take the FM radio spectrum from 88 to 108 MHz; each channel has to be separated 100KHz from the next and previous ones, therefore one can have maximum 10 stations in 1 MHz, and that spectrum is 20MHz wide. The total is 200 "slots" in which fit all transmitters. Now thankfully at these frequencies radio waves don't propagate easily beyond line of sight, compared to HF which benefits from ground wave and sky wave propagation (allowing for example foreign stations reception), so -roughly- each moderately distant city can have its own 200 frequencies free of interference from the next city (state, country). Still, we have only 200 possible stations in a place where there could be thousands of people willing to set up their FM radio, which of course is not possible. Hence the need to regulate the resource.