THE UNFORTUNATE decision by the Justice Department to label three Canadian documentary films "political propaganda" involves two films about acid rain and one, which has been nominated for an Academy Award, about nuclear disarmament. Not one of the three threatened the box-office reign of "E.T." But now many otherwise indifferent people will want to see them to learn what the fuss is about. What is so threatening about these documentaries that they must be labeled and shown only with a disclaimer that the U.S. government does not approve of them? Why on earth should the Justice Department need, want or be given a list of those to whom the film is shown? What country is this? What decade?

Dozens of films, American and foreign, could be characterized as political propaganda of some sort. But this society thrives on free political discourse. It is essential to the democratic form of government we have chosen and are constantly trying to perfect. And the taking of names of citizens who express or even choose to listen to unpopular argument is absolutely contrary to the values we hold as Americans.

Yes, the Canadians believe that particles from our smokestacks are killing fish in their lakes, and many in the American government don't believe the case has been proved. And certainly there is much to be said on every aspect of nuclear disarmament, and the film in question takes one point of view. But the way to counter "political propaganda" is to add to the dialogue, not stifle it. The State Department, for example, has good reason to believe that another picture nominated for an Academy Award, "Missing," is an inaccurate account of events during the coup in Chile. The department countered by issuing press releases and otherwise vigorously arguing its case. Why can't this method be used with films made by foreign governments or their agents?

The Foreign Agents Registration Act allows all the actions taken by the Justice Department in these cases. According to the department, at least 25 films distributed by agents of foreign governments are reviewed each year and about half are found to be "political propaganda." There's nothing wrong with notifying viewers of these films that they have been prepared by foreign governments, or with our government's presenting another side of the story. But labeling films as "propaganda" and keeping lists of viewers are disturbing and intimidating practices. The law was written in a different era. Surely we now have enough faith in ourselves and enough reverence for the First Amendment to change it.