I think you're getting at something fairly significant here - and the resolution is not at all obvious. First off, I personally think the defendant is in the right here - the logos are clearly political in nature, and I think they should fall clearly under 1st amendment protection. Whether or not that will happen, of course, is a different question.
Now, the analogy I'd like to make is to the case that Nike is currently involved with in, I think, Oregon or California. They responded to some attack ads about their business practices with their own version of the story. The people who put out the attack ads found some error or other, and sued under a law banning false advertising, claiming that it was commercial, not political, speech. I disagree with them, again, but their case rests on the idea that a private, commercial company only engages in commercial speech.
And that seems to be at the root of this case as well. Fox is, I assume, trying to maintain that they are a commercial company, and this parody is commercial, not political, speech. Especially for a news outlet, I find this tenuous - maybe laughable. Fox and CNN regularly engage in political speech, so do a host of other companies. When they do, they deserve the protection we afford such speech. But they should also take the blows, that other people can engage in such against them.
These t-shirts are attacking Fox and CNN's political views. The original writer may not find them clever, but personally, I did. I don't agree with them, but I thought they were amusing. More, I thought they were very clearly political themselves. Thus, I think they deserve a very high degree of protection.
Drew Garrett