IT'S EASY to imagine the frustration of the State Department as it watches official Soviet spokesmen hang out the Kremlin line time after time on American television while American spokesmen try repeatedly and in vain for comparable exposure on Soviet screens. It must have seemed an appropriate thing to invoke the principle of reciprocity and to deny a visa extension to a Soviet Central Committee member, Georgi Arbatov, who is already visiting in this country, in order to keep him from taking part in a PBS debate on April 10. At the same time visas were offered to two other Soviets invited by PBS for that program.

It was, nonetheless, a clumsy decision. It blurs the line that must exist, on the American side, between government and the press. The State Department, for its own official purposes (to pry American officials onto Soviet TV,) is intervening in the American journalistic process. It is thwarting a programming decision made by PBS. Not surprisingly, the two other Soviets invited to appear on the show say they will not accept an invitation linked to the forced disinvitation of their colleague.

The result is that American viewers are to be deprived of what promised to be an interesting exchange: a debate in which three ranking Soviet spokesmen would be challenged by three knowledgeable Americans on issues of highly topical interest. And the Soviets, who can claim that the American government is censoring an American program for an American audience, are being handed a mini propaganda coup.

Reciprocity is the correct principle to apply in East-West relations. It provides leverage in situations where Soviet performance would otherwise lag. But the principle cannot be applied inflexibly without regard for the differences between the two great powers. It can be irritating to see the Soviets taking advantage of such differences, as they are doing in this case. But it is best for the United States to rise above it and to take advantage of some of the differences itself. That entails sending Soviet citizens the American message on the international radio stations. Few Americans listen to Soviet shortwave broadcasts, but millions of Soviets tune in to the American ones.