Why is this legal?

why are horse races legal...i heard something on the radio that angered me deeply but then again saddened me...well the day b4 the Kentucky Derby i heard there was gonna be the first female horse in the race in a decade...and then today i heard they had to euthanize her because she broke 2 ankles during the race...am i the only one that gets so mad over this? i think thats TERRIBLE to push that horse so bad to win just for her to die...i do not own horses but i grew up around them and they are beautiful creatures...i dont understand why people do this to them?