I'm not just talking about a hijacked airplane, but a hijacked airplane that was about to be flown into a building in New York City. The airplane was flying east about 50 miles east of the NYC. How would we know it "was about to be flown into a building in New York City"?
Or are you saying that it was the policy of our government to shoot day every hijacked airplane at that time?
My theory is that there may have been a credible threat at the time (intruder in the cockpit indicating his intentions, or some kind of intelligence offering a clear indication of what was going to happen).