If the horse can be bred from, a broken leg is not necessarily a problem. If, however, it is a gelding, the brutal truth is that it's cheaper to destroy the horse than spend a lot of money on a horse that will not do anything more until the day it dies naturally. Not sure about the morality of that decision. Perhaps a tax on each bet could be siphoned off to care for such horses but racing is a business and most business men are not that sentimental.