Ok, I'm going to put another spin on this issue.
Horse racing, horse riding in general, is unnatural to the horse. But of the 18,000 or so horses sent to slaughter there are at least that number in work ready to be raced. So if horse racing was banned what would happen to the now 36,000 odd horses? Let alone broodmares, breeding stallions, weanlings and yearlings?
You have to look at the whole picture. Instead of banning racing completely look at trying to change the practices. For example, the barefoot movement. Lets try and get that pushed through and accepted by the industry. Then we can look at diet, equipment (bits) and the issue of the horses not making the grade.
And there has been some improvement. Jockeys are only allowed to hit a horse with a whip so many times during a race (and this is very heavily monitored), the horse only has to be shod in a quarter aluminum shoe now and not the whole shoe and ex racehorses are highly sort after as mounts for other discplines. Especially showing, eventing, Pony Club and ARC. Just have a look at Horse Deals. They're fetching upwards of $3000! Not bad for horses that otherwise would only fetch a few hundred $$ in the sales ring a few years ago.
Yes, its all good to go out protesting against the 'cruelty' of horse racing, but there are so many supporters of it, they will just see it all as troublemaking. So use some people skills, protest for some better changes instead and believe me you will get a lot further.
I have worked in a few race stables and its not as bad as it has been made out to be. The stablehands get first pick of the horses that are no good before they are moved on. They do get turned out on weekends or spelled for a few months. Yeah, the diets could be better, but they have improved somewhat in the few years that I have been around them. And a lot of these horses genuinely love to run. It's what they have been born and bred to do.
Don't condone an industry entirely before you have had experience in it. Anyone can write something on a website and make it seem the worst thing in the world. Just keep that in mind