“Typical rebels” might be unlikely to be able to shoot down a high-altitude plane. But the rebels in Eastern Ukraine had just demonstrated that they could shoot down a plane at 21000 ft. With that information who would conclude that there was less than a 1% chance the rebels were capable of downing a plane at 33,000 ft?

And even the most well-trained military force in the world has shown that it can mistakenly target civilian planes: http://en.wikipedia.org/wiki/Iran_Air_Flight_655