In a belated affirmation of the First Amendment, the Federal Communications Commission has cut broadcasters loose from that bureaucratic apron string known as the ''fairness doctrine.''

The fairness doctrine, a commission-made complex of regulations in force since the late 1940s, required broadcasters to provide ''contrasting'' views on issues of public importance. There are few mourners at its passing -- but there are some, chiefly in the world of politics and public-interest lobbying. The fairness doctrine discouraged adventurous and provocative public-affairs programming.

Congress tried two months ago to write the discredited doctrine into statutory law. Congress alone seems to take the view that the First Amendment ("Congress shall make no law . . . abridging the freedom of the press'') carries an unwritten rider: ''Except when exercised by electronic means.''

The old legal justification of the fairness doctrine was that channels and frequencies are ''scarce,'' hence must be rationed and monitored in the public interest. From a strict First Amendment point of view, that justification was dubious enough. In any event, cable (and more recently satellite transmission) long ago knocked the scarcity rationale into a cocked hat.

There is today no meaningful theoretical limit on the number of television channels available to any given viewer. Like El Cid, the great Spanish warrior, the fairness doctrine had been dead in the saddle long before the death was admitted and announced.

Does the story end with its belated burial? Not altogether.

The FCC dressed its axing of the fairness doctrine in high-flown constitutional rhetoric, invoking ''an uninhibited marketplace of ideas in which truth will prevail'' as the goal of First Amendment policy.

And in fact virtually every critic of the doctrine insisted that it had a ''chilling effect'' on the airing and discussion of public issues, since the safest way to stay clear of it was to be innocuous. It sought variety but promoted blandness. A 1983 FCC study found ''over 60 specific examples of chilling.'' It also found that many stations were shying away, as a matter of policy, ''from controversial issues.''

Still, it would be a mistake to see the issue as a straight-out clash between heroic broadcasters yearning to be free and provocative and smothering FCC bureaucrats peering over their shoulders.

The fact is that although television and radio long to be accorded grown-up First Amendment privileges, and should be, broadcast journalism is still troubled by the unadult practices of the industry.

Broadcasters long ago made the awful mistake of surrendering to advertisers ("sponsors,'' in the evasive language of this overcommercialized industry) the privilege of dictating program content and time slots. The practice devalues the ''journalistic'' ingredient in broadcasting.

In print journalism there is no such slavery to advertisers or to ratings. Few newspapers and magazines could honestly claim to be wholly free of the influence of powerful advertisers on content and placement, now and then. But they do not dictate either. There is a vast difference between an occasional exception to a sound rule (that editors determine content and its placement) and the usual broadcasting practice of allowing ''sponsors'' routinely to call the shots.

The result is that broadcast journalism probably suffers more from the influence of advertisers on the front office than from the ''chilling'' influence of FCC regulation. That will continue to be the case, even under the new rules, so long as money and ratings push public-interest programming out of the way for quiz shows, sitcoms and other diverting trivia. In this frivolous environment, freedom from FCC regulation is not likely to be the emancipation it could and should be.

The FCC has, in short, made a much overdue constitutional correction. But the beneficial effects will emerge only if and when broadcasters decide they have a worthier journalistic mission than ''entertaining us to death.''