This happens so often, but is rarely in the public eye.
For racing purposes, all horses turn a year older on January 1 of each year, regardless of their actual birth date. Breeders largely use hormones to fake out the mares into cycling to get foals born as close to January as possible, otherwise, you may have a foal born in April or May, and when you're talking 20 months old compared to 24 months old, in a horse, that's a big difference.
Anyway...since the Derby is a "championship race" based on earnings, these guys are running their hearts out at the ripe old age of 2 ("figured" age, not actual age), which means they're training when they're 1. Any good vet will tell you NEVER to run your horse before 4. And these guys are training at 1. Any good trainer will even admit its a fine line between physically ruining the horse forever and a good race. Its just ugly.
Another thing that bugs me...if she'd been a colt instead, they likely would have tried to save her. Colts can sire many foals and make more money. Its a brutal industry.