Frankly I wouldn't let the Soviets off that easily on that basis. The behaviour of the flight that was not consistent with previous American reconnaissance missions should have tipped the Soviets off, and they had courses other than shooting down the jet (Put it this way: Technically in some US jurisdictions it is legal to brandish a firearm at or shoot some kid that intruded on one's property. Nonetheless the shooter shouldn't expect anyone to break out the 18-year old scotch for him at the next block party) per David E. Hoffman's chapter on KAL 007 in "The Dead Hand":
<<Another plane flew in the sky that night, circling close to the Soviet Union, an RC-135 four-engine jet used for intelligence missions by the U.S. air force. The RC-135, a converted Boeing 707, was a familiar spy plane, known to the Soviets. Osipovich, the interceptor pilot, recalled he had chased it many times. The RC-135 flights were monitoring Soviet ballistic missile tests on an intelligence mission known as Cobra Ball. The plane was crammed with cameras and special windows down one side to photograph a Soviet missile warhead as it neared its target. The upper surface of the wing on the side of the cameras was painted black to avoid reflection. The RC-135s were based on Shemya Island, a remote rocky outcropping in Alaska’s Aleutian Islands.
Soviet missile tests often aimed at the Kamchatka Peninsula. How the missiles landed could help the United States monitor arms control treaties and look for violations. The pictures could show how many MIRVs came from a missile and the final trajectory. The RC-135 planes flew in circular or figure-eight orbits with camera lenses aimed at the Soviet coastline, in anticipation of a test. On the night of August 31, a missile test was expected and the RC-135 loitered in the sky, waiting. The RC-135 had a wingspan of 130 feet, compared to the 747, which stretched 195 feet and 10 inches across. Both had four engines, located under the wings. The 747 featured a prominent hump on the front of the fuselage for the upper passenger deck. As the RC-135 circled, at about 1 A.M., the larger 747 flew by, seventy-five miles south. This was a critical moment of confusion for the Soviets. They had been tracking the RC-135 by radar. When the missile test didn’t happen, the RC-135 headed back to its base on Shemya Island, but Soviet radar didn’t see it turn and go home. On the way home, the RC-135 crossed the flight path of the 747 at one point. The Soviet radar somehow lost the RC-135 and picked up the 747, now unexpectedly heading directly for Kamchatka. The plane was given a number, 6065, and the track was annotated with an “81,” which meant one unidentified aircraft. It was the off-course Korean Air Lines flight, but the Soviet ground controllers thought it might be an RC-135. The radar tracked the plane as it approached Kamchatka, but not constantly. Radar contact was lost, and picked up again while the plane was about halfway over the peninsula.
When the airliner approached Kamchatka, Soviet air defense forces were slow to react. Controllers were groggy, commanders had to be awakened, and there were radar gaps. Transcripts of ground control conversations show they spotted the plane just as it flew over the air defense forces base at Yelizovo. They scrambled four interceptors. These planes zigzagged in the air for twenty minutes but could not find the jet, which was actually north of them, and they were forced to return to base. The plane flew on, straight out over the Sea of Okhotsk and toward Sakhalin Island, about seven hundred miles away. Radar contact was lost at 1:28 A.M.
the plane just as it flew over the air defense forces base at Yelizovo. They scrambled four interceptors. These planes zigzagged in the air for twenty minutes but could not find the jet, which was actually north of them, and they were forced to return to base. The plane flew on, straight out over the Sea of Okhotsk and toward Sakhalin Island, about seven hundred miles away. Radar contact was lost at 1:28 A.M.
Guk, the KGB chief in London, had been in Moscow during the shoot down, and he later took Gordievsky aside and told him that eight of the eleven Soviet air defense radar stations on Kamchatka and Sakhalin were not functioning properly. Dobrynin heard>>
Emphases seem to corroborate the testimony of the Soviet defector pilot Alexander Zuyev, who claims that several EW and GCI radars remained inoperable because they hadn't been repaired in a timely manner despite the Far East MD's claims to the contrary.
<<At 3:09 A.M. an order was given to destroy the plane, but then rescinded. The Sokol command post duty officer wondered if the Americans would really fly a spy plane directly into Soviet airspace. They usually circled outside territorial waters. “Somehow this all looks very suspicious to me,” he said. “I don’t think the enemy is stupid, so … Can it be one of ours?” He called another command center at Makarov, on the eastern tip of the island, to see what they knew about the plane’s flight. “It hasn’t bombed us yet,” was the reply.
The Soviet ground controllers asked Osipovich six times whether the airliner was showing navigation lights, on the assumption that a plane without them might be on a spy mission. At 3:18, Osipovich reported, “The air navigation light is on, the flashing light is on.”>>
Note that contrary to popular culture examples even the SR-71 kept their flights outside of Soviet airspace, as they had sensors and photographic equipment powerful enough for their missions monitoring Soviet naval bases in Murmansk, Vladivostok, and Petropavlovsk from this distance.
<<At 3:24, Osipovich’s radio crackled with orders:
“805, approach target and destroy target!” The airliner was just slipping away from the Sakhalin coast. Osipovich recalled later it was at this point he had finally gotten a look at the plane, and he realized suddenly it was larger than an RC-135. “Soon I could see it with my own eyes,” he recalled. “It was a big plane, and I thought it was a military-cargo plane because it had a flickering flash-light. There were no passenger plane routes, and there had been no occasions of any passenger planes losing their way…I could see it was a large plane. It wasn’t a fighter plane, but either a reconnaissance plane or a cargo plane.”
Dobrynin recalled seeing Andropov that day. Looking haggard and worried, Andropov ordered Dobrynin to rush back to Washington to deal with the crisis, saying, “Our military made a gross blunder by shooting down the airliner and it probably will take us a long time to get out of this mess.” Andropov called the generals “blockheads” who didn’t understand the implications of what they had done. Dobrynin said Andropov “sincerely believed,” along with the military, that the plane had made an intrusion into Soviet airspace as part of an intelligence mission to check Soviet radars. But even that, Andropov said, was no excuse for shooting it down instead of forcing it to land.>>
The PVO commanders made a stupid gamble for the sake of their careers ordering the shootdown of KAL-007.