The fairness doctrine was a Federal Communications Commission (FCC) policy. The FCC believed that broadcast licenses (required for both radio and terrestrial TV stations) were a form of public trust and, as such, licensees should provide balanced and fair coverage of controversial issues. The policy was a casualty of Reagan Administration deregulation.
The Fairness Doctrine should not be confused with the Equal Time Rule.
This 1949 policy was an artifact of the predecessor organization to the FCC, the Federal Radio Commission. The FRC developed the policy in response to the growth of radio ("unlimited" demand for a finite spectrum lead to government licensing of radio spectrum). The FCC believed that broadcast licenses (required for both radio and terrestrial TV stations) were a form of public trust and, as such, licensees should provide balanced and fair coverage of controversial issues.
The "public interest" justification for the fairness doctrine is outlined in Section 315 of the Communications Act of 1937 (amended in 1959). The law required broadcasters to provide "equal opportunity" to "all legally qualified political candidates for any office if they had allowed any person running in that office to use the station." However, this equal opportunity offering did not (and does not) extend to news programs, interviews and documentaries.
Supreme Court Affirms Policy
In 1969, the U.S. Supreme Court unanimously (8-0) ruled that Red Lion Broadcasting Co. (of Red Lion, PA) had violated the fairness doctrine. Red Lion's radio station, WGCB, aired a program that attacked an author and journalist, Fred J. Cook. Cook requested "equal time" but was refused; the FCC supported his claim because the agency viewed the WGCB program as a personal attack. The broadcaster appealed; the Supreme Court ruled for the plaintiff, Cook.
In that ruling, the Court position the First Amendment as being "paramount," but not to the broadcaster but to the "viewing and listening public." Justice Byron White, writing for the Majority:
The Federal Communications Commission has for many years imposed on radio and television broadcasters the requirement that discussion of public issues be presented on broadcast stations, and that each side of those issues must be given fair coverage. This is known as the fairness doctrine, which originated very early in the history of broadcasting and has maintained its present outlines for some time. It is an obligation whose content has been defined in a long series of FCC rulings in particular cases, and which is distinct from the statutory  requirement of 315 of the Communications Act [note 1] that equal time be allotted all qualified candidates for public office...
On November 27, 1964, WGCB carried a 15-minute broadcast by the Reverend Billy James Hargis as part of a "Christian Crusade" series. A book by Fred J. Cook entitled "Goldwater - Extremist on the Right" was discussed by Hargis, who said that Cook had been fired by a newspaper for making false charges against city officials; that Cook had then worked for a Communist-affiliated publication; that he had defended Alger Hiss and attacked J. Edgar Hoover and the Central Intelligence Agency; and that he had now written a "book to smear and destroy Barry Goldwater."...
In view of the scarcity of broadcast frequencies, the Government's role in allocating those frequencies, and the legitimate claims of those unable without governmental assistance to gain access to those frequencies for expression of their views, we hold the regulations and  ruling at issue here are both authorized by statute and constitutional.[note 28] The judgment of the Court of Appeals in Red Lion is affirmed and that in RTNDA reversed and the causes remanded for proceedings consistent with this opinion.
Red Lion Broadcasting Co. v. Federal Communications Commission, 395 U.S. 367 (1969)
As an aside, part of the ruling could be construed as justifying Congressional or FCC intervention in the market to limit monopolization, although the ruling is addressing the abridgment of freedom:
It is the purpose of the First Amendment to preserve an uninhibited marketplace of ideas in which truth will ultimately prevail, rather than to countenance monopolization of that market, whether it be by the government itself or a private licensee. It is the right of the public to receive suitable access to social, political, esthetic, moral and other ideas and experiences which is crucial here. That right may not constitutionally be abridged either by Congress or by the FCC.