Why do racehorses with injured legs have to be put to sleep?

Any equine experts here?

Leg injuries that are not immediately fatal still may be life-threatening because a horse's weight must be distributed evenly on all four legs to prevent circulatory problems, laminitis and other infections. If a horse loses the use of one leg temporarily, there is the risk that other legs will break down during the recovery period because they are carrying an abnormal weight load. A horse cannot simply lie down in the equivalent of a human's "bed rest"—an animal of this size will literally crush the internal organs when lying supine for extended periods. If a horse loses the use of one leg for a long period, its other legs will ultimately break down as well, with euthanasia the only possible outcome.

http://en.wikipedia.org/wiki/Thoroughbred#Difficulties_treating_injured_horses