FCC, especially for radio. It costs something like 100K just to APPLY for a radio station licence. If a channel isn't being used, it should be free to homestead -- just like other property. This would result in an explosion of free speech.
We have a bunch of homestead-able spectrum in New Zealand. It makes it mission-critical that your transmitter doesn't go off air for even a moment or someone else grabs the bandwidth insanely fast an boom your are out of business.
But yes the limited nature of the radio spectrum makes secured free-to-air bandwidth incredibly powerful.