I want to say it was a clause of the FCC’s Fairness Doctrine, wherein newscasters were legally required to present information on events and political matters as unbiased as possible on publicly-issued airwaves. It lost a lot of its steam when cable became commonplace, as cable networks were technically closed-circuit systems, and then it went out the door with the internet. On top of which, stations like Fox News claiming to be entertainment and not news stations helped their cause. The original idea was that if the FCC was to grant you a broadcast license, you were obliged to operate in the interest of the public, and the doctrine expressly forbade operating for personal gain.