An inadvertent shootdown during an intercept is far from unheard of.

These absurd NORAD theatrics that put live air-to-air missiles in close proximity to airliners are just insanely poor risk management.

Besides, from a practical and moral standpoint, it's all but impossible to envision a scenario where cold-bloodedly shooting down an airliner is the right answer. Letting the threat of terrorism intimidate us into making Sophie's Choice is a pure win for evil.