The radio station is private property and a private company
There was a DJ fired from an NPR station a few weeks ago. He said he was fired for speaking in support of the war. The manager said he was fired for not playing the NPR news. Either way, a private employer can legally fire an employee for violating company policy. The First Amendment applies to government, not to the private sector. If you don't believe it, go up to your boss and cuss him out. See how long you get to keep your job on "free speech" grounds. Odds are it won't be long.