It's actually a fairly common belief - amongst people who don't know horses - that racing them is inherently cruel.
Racing is actually the least unnatural of the various things we ask horses to do. Not all horses enjoy a race - but most do. If you line horses up at one end of a field, look at the other and yell go, they'll race - and they'll get quite competitive.
Running bred horses are, of course, selected for this competitive instinct. A quality racehorses knows his job and enjoys his job - getting around the track first. Horses are a flight animal, and race "play" probably prepares them for outrunning predators.
(This is not saying there aren't abuses in the racing field, but inherently, asking horses to race isn't cruel at all - it's natural play for them. They'll do it turned out in the pasture).