Ted Kennedy's furtive stiletto job on the Boston Herald shows that the senator's patience with press freedom is not inexhaustible. But by focusing overdue attention on a consummately stupid policy, it serves a good purpose.

The Federal Communications Commission has given Rupert Murdoch, the Australian-born press magnate who owns the Herald, a waiver of its ''cross-ownership'' rule. That 1975 rule says that you can't own both a paper and a TV license in the same market; and Murdoch does. Without the waiver, Murdoch would be forced to dump the shaky Herald and another ailing paper he also owns, the New York Post.

It happens that the Herald has been tucking it to Teddy. So the senator persuaded his friend Sen. Fritz Hollings to insert an unnoticed provision into the recent omnibus budget bill. The provision bars any extension of FCC waivers. It blocks Murdoch's waivers, due to expire this spring, and thus puts both papers, especially the Post, in danger of closing.

New York Mayor Ed Koch, a Murdoch favorite, accuses Kennedy of a ''sneak attack'' on the First Amendment. And indeed any act that contributes to a newspaper's demise (even that of the scandal-peddling Post) is First Amendment vandalism. But Koch is far off the real point. The blame should be fixed where it belongs -- on the fatuous old FCC, only begetter of the cross-ownership rule.

Speak the shibboleths ''antitrust'' or ''antimonopoly'' in Washington and goo-goos swoon. As a policy principle it stands next to cleanliness if not godliness. Yet the fact is that no more counterproductive communications policy has been concocted in decades, if ever.

When television was in its infancy, its lush profits unforeseen, the FCC encouraged newspaper owners to become TV licensees. Many did. Having thus fostered a host of cross-ownerships, the FCC suddenly decided in the mid-1970s that the policy was wrong and issued its anti-cross ownership decree. Ultimately, after the courts struck down its attempt to grandfather incumbent cross-owners, newly designated media monopolists were able to solve the problem by swapping licenses from city to city.

The weak suffered, as I have personal reason to know. In the spring of 1975 Joe Allbritton, the new owner of The Washington Star (of which I was editorial page editor) was seeking just such a waiver as Murdoch now enjoys.

The Star was failing, and Allbritton proposed to divert revenues from the Star Company's money-minting Washington TV station (now WJLA-TV) to revive it. As The Star bled, there followed snail's-pace hearings in which some FCC commissioners seemed to find it hard to follow the case or grasp its possible consequences. Finally, the FCC denied Allbritton a waiver.

And having forced him to sever the two properties, it even insisted that he quit as unpaid titular publisher of the Star, which he had sold to Time, Inc. Despite Time's heroic efforts, The Star was soon dead. And the pawprints of the FCC's anti-monopoly tigers were all over its throat.

Thus did a rule designed in theory to promote competition contribute substantially to the death of competitive newspaper journalism in the nation's capital, which needed it then and needs it still.

This is obviously a self-interested complaint, but I offer it unblushingly. The FCC's obtuseness still angers me and would, I trust, anger me if a paper other than my own had been the victim. The cross-ownership rule is a brilliant study in counterproductive policy. But in Washington, good intentions are often allowed to excuse calamitous real-world consequences.

The FCC isn't the only accident-prone federal regulatory agency, but it seems to have a special talent for growing thumbs. Recent example: In the name of ''deregulation,'' the Reagan FCC unleashed the networks to clog prime time with commercials, as many as the traffic will bear. If this eventually proves to be the death warrant of network television, it won't be the first time the FCC's wonderworks have backfired.

The parties to the entertaining Kennedy-Murdoch/Herald-Post battle are fighting over trivialities. They should band together and get this stupid rule off the books and off the backs of American newspapers. It helped break The Star's and it will surely break others.