Turok and TM both make good points. Turok is absolutely right that there is a difference in the right to free speech and the consequences of it. TM is right that those consequences seem to fall disproportionatley on those who violate the dictates of political correctness. But I think their points, although valid, are not what is at issue here.
There are many constitutionally appropriate restrictions on free speech, the right to cry "Fire" in a theater being the classic. The Supreme Court has long recognized that broadcasters do not have the same rights that publishers do. Broadcasters are allowedto use the public airwaves, ie the limited frequencies available for broadcasting, and in exchange they have an obligation to act in the public interest, broadly defined. Publishers have no such duty. Anyone can start a newspaper and give it away or sell it. You have to be granted a license to operate a radio or TV station.
So the issue regarding what broadcasters are allowed to send out over the public airwaves is not strictly a free speech question, although clearly there are substantial free speech overtones. For me, the better analogy is to zoning. Zoning restricts your property rights, but the theory is we accept that tradeoff for the greater good. The FCC applies a type of zoning to broadcasting. If it is done properly, we are all better off. We can allow our kids to watch TV or listen to the radio with a certain confidence. If broadcasters feel there is a market for filth, they can use cable or the internet.
What about Clear Channel ditching Stern, supposedly to curry favor with the White house? As TM suggests, anyone who has a problem with that should be protesting ESPN for dumping Limbaugh or the networks for dumping Dr. Laura. They have a right to select whom they want, and there is no free speech issue involved.
Could the FCC regulate Rush Limbaugh out of existence? Yes, probably so, but they would have to do it in such a way that all political advocacy was eliminated. They effectively did that for a long time with the so-called Fairness Doctrine. Finally it was scrapped. It was a joke because it kept the Rush Limbaughs off the air but did nothing to eliminate the subtle propagandizing of Walter Kronkite or the pervasive liberal bias that has made network news such a joke.