i think the reason they shoot horses who have a broken leg or two is that they constantly have to be on their four legs. there's no way to keep a horse off it's feet unless you chain it down or something, but that is bad for them too.
since there would be constant weight on the injury, i think the bone would have a hard time healing. i heard this from my sister who is about to graduate from vet school.
*i never have or will support animal exploitation. something always happens at the expense of the poor animals.