As bubba28 says, it depends on the type of fracture. Small chip fractures, or small fractures in certain bones, can be repaired. Big nasty ones often can't. Horses aren't very good patients for fracture repair as they get infections into the fracture site very easily. They're huge animals on tiny legs (horses walk on one toe!) and don't cope well with one leg out of action. Often the leg on the other side develops a problem if the horse puts all its weight on it.
For most fractures an attempt can be made to fix it. But certain fractures, for example comminuted fractures where the bone has shattered, usually have a hopeless prognosis and aren't repaired. However it is very expensive to fix a fracture in such a large animal - the time, loss of performance, equipment like slings to hold the horse upright, and veterinary costs - and even after putting all that effort in one can't guarantee that the broken bone is actually going to heal. There's also the case of damage to the other leg while the horse is recovering. In many cases the horse won't return to full function afterwards.
In A Pickle, for many racehorses, the fracture occurs during the race and is traumatic in origin - often you get comminuted fractures (poor prognosis) and there may also be an open wound over the site (even poorer prognosis). Unless it's a very valuable racehorse and there's a chance of recovery sufficient for breeding, the best option for the horse is euthanasia.
The same applies for many pet horses - the cost of treatment can be so high that the owners simply can't afford it. For me euthanasia is a perfectly acceptable option and owners shouldn't feel bad about not being able to pay.