Ollie, no offence but that is naive.
Wars with fast jets involved are not nicely compartmentalised above and below 20,000 ft. Wars are messy and confused and things change rapidly - notice how the rebels did not issue a NOTAM saying they'd just acquired some fantastic new russian kit. And even if you were safe above 20,000 - what happens when you depressurise and dive down to 10,000? I hope you wouldn't fly over a mountain range thinking 'oh that wont happen anyway - not an issue'
Don't you see - its precisely that kind of 'risk assessment' which has got the industry to where it now stands?
The Americans (the FAA) made the correct call. They had more intelligence data than anyone and were worth listening to.