BUK, as I understood it, fires at high altitudes, 10,000 metres, exactly where passenger aviation flies.
Ukrainian military planes flew 5,000-7,000 metres, within the range of the existing separatist arms. Proven by the loss of it all.
Who, in his right mind, would take aim at 10,000 metres where there is, technically, nothing military above Ukraine?
Whoever (if it were a BUK from the ground) took the aim high up there - what was in mind? To find there - what? on earth
From a distance of 30km, a plane at 6000m and one at 10000m appear at an angular distance of roughly 8 degrees to the man at the radar. If the 6000m plane is 12km closer (at 18km), they would appear in the same spot, the closer one hiding the higher-up, further-away one. If you're under pressure, that may not be so easy to sort out quickly!