there's two possibilities regarding this downed plane in ukraine.

a) the separatists accidentally shot it down, which would be very sad but is the consequence of flying into a war zone. the fact that this question (why didn't they detour?) isn't being asked is very telling. but, i'm not willing to escape occam's razor just yet.
b) it's the american government's most recent staged attack to launch a war.

the idea that the rebels would purposefully shoot down a foreign plane is laughable on it's face, and the moment you start hearing that you know the answer is (b).

and that is a frightening possibility.

just on the most basic issue of resources, the rebels in ukraine could not possibly think the lives of the people on the plane are worth the price of the missile to shoot it down. they have real targets to worry about.

"shit. the ukrainians just took the city."
"why didn't you shoot the helicopter down?"
"we used our last missile on a terrorist attack on a malaysian passenger plane, and don't expect more until
tomorrow."
"you idiots."
"appears to be the case, sir...."

i mean, shit happens. it could have been a botched shot. but it could not have been purposeful, and anybody suggesting otherwise is lying to you with a straight face.