The FCC and "Public Convenience, Interest, or Necessity"

Seven years later [1934], a new Federal Communications Commission took over this role [of granting radio station licenses]. Henceforth, licenses were granted for three years at a time to broadcasters who could persuade the FCC that their station would serve 'public convenience, interest, or necessity,' criteria that were never applied to newspapers. As a consequence, free speech on air was severely circumscribed by both regulators and (because of the importance of advertising as a source of revenue) commercial interests. - Niall Ferguson

Different rules were applied to radio stations than to newspapers and regulation DID impede free speech. The radio model was later applied to TV stations as well. The dawn of the Internet reduced this government and sponsor interference with free speech through the growth of blogs, podcasts, and YouTube (and other platforms) videos. Now big tech, in combination with corporate media, and the government is clawing back the defacto censoring capabilities the FCC for so long has wielded over radio and TV stations.

"Public convenience, interest, or necessity" could be interpreted to mean almost anything. And it has been interpreted as such largely depending on the political leanings of the regulators.