THE Soviets have at last freed Mathias Rust, the young West German aviator, as well they should have. His remarkable flight from Finland, culminating in a touchdown in Moscow's Red Square, embarrassed the guardians of the Soviet Union's air defenses, but gave Mikhail Gorbachev an opportunity to replace some of the military establishment with his own men. Young Rust is not being pardoned, but rather his four-year sentence is being cut short. This way the Soviets get to look tough, and still get points for humanitarianism, too. A desire to have the Rust matter resolved before West German Chancellor Helmut Kohl comes calling in October was presumably also a factor in the Kremlin's decision. But whatever the reason, the Soviets did the right thing. Welcome home, Mathias!