Actually, Ukraine is one of about a dozen countries that SA-17 missile systems, so the Ukrainian Army does have them. And given the amount of equipment that the pro-Russian separatists have gained from capture and defection from Ukraine forces and from seizure from warehouses in eastern Ukraine, there should be no surprise that the rebels also now have Grizzly systems. But neither side stands to benefit from shooting down a passenger airliner, so the most logical explanation is that the rebels mistook the Malaysian B777 for a Ukraine Air Force transport. I believe there has been at least one recent shootdown of a UAF AN-24 over rebel territory. Cui bono?
There is one problem with that. There is no way you can mistake a commercial target at over 30,000 feet for a cargo plane at 10,000 feet. Not possible. Visually there is a massive size difference and the altitude should have been displayed on the radar guidance system controls. And commercial planes, the ones flown by idiots, were traveling through that airspace at altitude throughout the day.