“The question is, why in the blazes does one shoot down a passenger plane? Target practice?”
The purpose of terrorism is to terrorize. There is (usually) no military reason to shoot at a civilian target. But this exercise may be to disrupt civil life in an effort to get civilians to work on their government to end the conflict. On the other hand, in World War 1 a problem with using messenger pigeons was the troops got bored and shot at them. (Or, shot them for food.) It may be that the guys on the ground got bored and decided to use one of their excess shoulder rockets.
Shoulder-fired rockets don't reach to 30K feet. The current thought is that it was a radar-guided rocket fired from a vehicle. At 30K, it probably takes a trained radar operator to distinguish between an airliner and a military transport, and it's unlikely the rebel operator was well trained.