The convoy was clearly marked and had been identified to the Russian command. So why did they bomb it?
My theory is that it was a Putin gesture of contempt. Putin believes that the West is weak and incapable of the resolve to resist. He cares nothing for the kinds of humanitarian considerations that are important to Obama and Kerry. He is testing the West, perhaps in preparation for his next military adventure - perhaps against the Baltics.
Of course if the Siberian candidate is elected, we can kiss them (and NATO) goodbye.