Why not a simple market solution? Assuming that there are more bids than there are minutes of available airtime, the station should sell the time to the person willing to pay the highest price.
Note that the individual discussed in the OP was not asking the stations in question to air his ads as a public service. As far as i can tell, he was willing to pay the market price to air his commercials. Clear Channel did not, as far as we know, deny his request because he did not have the requisite amount of money. And presumably, if they had denied his request because all the slots were full, they would have said so. As it is, they won’t even give a reason for their refusal.
It’s obvious that the answer to the OP’s question has already been found: the airwaves themselves are the property of the state, but the licensees are allowed to refuse advertising they don’t want to take. This much is clearly true under the current system. Personally, i tend to think that the licensees should be under a slightly greater obligation in terms of their duties to the public. In the arena of advertising, i think they should be obliged to take any advertisement that:
a) does not contravene FCC guidelines
and
b) is offered at the media outlet’s current rate of payment for the timeslot.
If these conditions are met, and two parties both want the same slot, they could bid against one another. If they were not willing to engage in a bidding war, the spot could go to the first advertiser to make a market-priced offer.
Of course, the practicalities of administering such a system wood be very difficult, because a station that didn’t want to air a particular ad could just arbitrarily bump the price of entry, or claim that some other advertiser had already booked the time-slot.
All issues of legality and property rights aside, i actually think that media outlets who really believe in freedom of expression should take ads from anyone. That’s why i had no problem with those student newspapers who took David Horowitz’s ads opposing slave reparations and were so roundly lambasted by liberals and leftists for doing so.
I subscribe to The Nation magazine, and over the past couple of years they have regularly run ads for the FOX News Channel. Quite a few subscribers were furious at this, and some cancelled their subscriptions. Personally, i was quite happy for the magazine to take FOX’s money while continuing to editorialize about what a dishonest and biased news service FOX is.