The KAL tragedy was in large part due to the fact that the plane penetrated not just "some" restricted air zone, but the one where the USSR
had had its major missile well array (Kamchatka is one of the parts of USSR
/Russia closest to the US territory). There were many speculations in the press at that time that the departure of the flight from Anchorage had been intentionally delayed to get a US satellite in place to monitor the reaction of the Soviet systems to the penetration event. I cannot comment on that. However, the fact that an unauthorized plane got close to one of the most sensitive areas of the Soviet airspace is impossible to deny.
That tragedy had consequences. When, some time later, the kid performed a flight from Finland to Red Square in Moscow, nobody was brave enough to give an order to intercept him. The full responsibility took Defence Minister or one of his deputies (I do not remember exactly), who was fired by Gorbachev.
Certain routes of the Soviet air space have been open for foreign planes for many years, most notably the routes from Western Europe to Far East and to South Asia (Minsk-Moscow-Aktyubinsk-Samarkand). Aktyubinsk (AKX, an otherwise unremarkable small Kazakhstan town) has a very good runway, and a foreign 747 landed there in emergency at least once). These routes were a good source of hard currency for the USSR
, and the government promoted them in every way. At first, the government insisted that the foreign planes on the Far East route would make a stop in SVO
(landing fees, refueling, shopping, etc.). When 747-400 emerged on the scene, such a stop has become even less necessary, which resulted in tough negotiation between the USSR
government and the airlines. As far as I understand, some compromise solution has been found, and many planes from Western Europe to Japan fly over the Russian territory nonstop nowadays.