Just thinking about the 2 horses put down in the national and I am sure either would have been saved if it was possible. Its clear that the owners of accoding to Pete would have spent whatever it took to keep him alive http://www.bbc.co.uk/news/uk-england-york-north-yorkshire-17719966 and I am fairly sure the same could have been said for Synchronised Dogs and Cats routinely recover from broken legs so I wonder why Horses cant?
I always thought that, just presumed they take the decision to do it because they can't race anymore so. No use to a trainer.
A Horse's leg would never heal sufficiently enough to be able to support it's vast weight, unfortunately.
It's because of a few things. In short they spend so much time on their legs that there is no real time to heal (a cat and dog lays down a lot, a horse doesnt), they have 80 bones in their legs which makes it more complicated and they don't spread their weight evenly over all four legs. http://animals.howstuffworks.com/mammals/broken-leg-horse.htm
According to a racing expert (trainer maybe) on radio on Friday, he was saying it's because "you can't tell a horse it will be ok", put a plaster on it's leg and the horse will just panic and perhaps be a greater danger to itself. Personally I think it's because the owners know it will never race again so don't want the expense.