Rush has a level of star power that puts him on equal footing with Clear Channel. He can't be pushed around by them. You see the same thing with entertainers who've risen to superstar status. They're powerful enough to have a 'seat at the table', and can even call the shots on major projects.
They need him a lot more than he needs them. He could even walk away if he wanted, and build a new network. His audience, the advertisers, and the investment dollars would follow him.
I don't think anyone's pressuring him one way or the other about his editorial content.
In any case, his "star power" is pretty much limited to politically interested conservatives -- and not all of them.