WHAT IS IT that accounts for the crudeness so often evident in the Soviet government's dealings with foreigners, even - especially - on issues that don't much matter? We have no good explanation but we take note, in a mood of sadness tinged by anger, of two recent episodes in this vein.

In one, a Korean Air Lines plane that had strayed far off course over northern Russia was shot up by a Soviet interceptor. Two passengers were killed and others wounded; all 100-plus aboard could easily have been lost. Why did the Soviet pilot shoot up an unarmed civilian aircraft? Is that what Soviets regard as vigilance? Or bravery? The authorities, saying that they are investigating, have yet to release the Korean pilot and navigator. But it is Soviet performance that ought to be investigated. It was inexcusable.

In a second incident, Soviet cops roughed up a Soviet woman and her daughter who were demonstrating outside the American Embassy to publicize the Kremlin's refusal to let them join her American husband, Woodford McClellan, of four years. The Soviets then refused to transmit photos and television film of the demonstration, relenting only a day later. It is heartless of the Russians, and contrary to their pledges in the Helsinki agreement, to keep the McClellans apart. It is repressive, and similarly contrary to Helsinki, for them to interfere with the reporting of news. If that is what the Russians intend to do when the international press corps hits Moscow for the 1980 Olympics, then the United States and other Western countries should be building a structure of precedent to deal with the press incidents likely to arise at that time. Meanwhile, the Kremlin must be kept under steady pressure to permit the reunification of families split by the Soviet frontier.

It is, some say, unhelpful to criticize the Russians on these matters: It gets their pride up, it recalls the cold war. Nonsense. When people are needlessly killed or injured, when their lives are broken, then pointed criticism is fair, morally necessary and, in our judgment, politically useful in order to encourage more responsible behavior the next time around. These incidents do not involve sensitive internal affairs in which the claim of outsiders to interfere is debatable. They impinge directly on foreigners. If foreigners don't speak up, the crudities will go on.