CBS and NBC have refused to broadcast a 30-second commercial produced by the United Church of Christ, because it's "too controversial": It basically points out that the UCC, unlike many other denominations, welcomes gay couples. So the UCC has filed petitions with the FCC to deny license renewal to a couple of CBS/NBC affiliates in Miami.
Now, I like the UCC commercial, and I agree with their message. I don't particularly like the idea of forcing networks to sell commercial time and air ads they don't want to air.
I do find CBS's policy a little baffling: They say they won't accept "advertising that touches on and/or takes a position on one side of a current controversial issue of public importance" (funny, I could swear I remember seeing ads fitting that description on CBS in the weeks leading up to the election). I also find it hard to believe they never run "advertising that proseletyzes on behalf of any single religion".
So it seems to me that they're being rather arbitrary in refusing to accept this particular ad. In a perfect libertarian utopia, though, they would have the right to be arbitrary. And the fact that you're reading this is an example of how that would be OK - the UCC is still getting their message out through other means, and the CBS/NBC refusal to air even generates publicity itself.
In our non-utopian world, the legal argument might have some merit. Basically, in earlier rulings, the USSC relied on the existence of the fairness doctrine to say that although there is some requirement that FCC licensees (TV stations) serve the public interest, including discussion of issues of public importance, networks couldn't be forced to sell air time. But the UCC argues that the FCC has stopped enforcing the fairness doctrine, so the court needs to re-examine the issue, which I would agree with, based on the case law cited in the petition.
I don't think TV stations should be under any obligation to "serve the public interest", but if we're going to insist that they are, then I agree that rejecting an ad specifically because it takes a position on an issue of public importance fails to uphold that obligation.