I'll take the usual "depends on definition" stand.
In this case, what kind of racing are we talking about and by what rules it's ran.
There's flat racing, jump racing, harness racing (the pretty much only form in Finland/Nordics) and endurance racing.
I'd say that it becomes cruelty when there's a high risk of injury to the animals.
(quoted) Mongolian Groom Racehorse Dies at Santa Anita Park, Raising Death Toll to 37 Since December
3 November 2019
Mongolian Groom was euthanized Saturday after injuring his left hind leg in the stretch of the $6 million Breeders’ Cup Classic at Santa Anita Park, becoming the 37th horse to die at the California track since December.
The 15-1 long shot was part of the early pace in the 1 1/4 –mile race, but appeared to take a bad step before he was pulled up and eased by his jockey during the stretch run of the race.
In front of a crowd of more than 67,000 spectators, the 4-year-old racehorse was loaded onto an equine ambulance and taken to the hospital, where he was later euthanized after a team of four veterinarians determined he couldn’t be saved.
On Saturday, a 37th racehorse died at Santa Anita Park since December as Mongolian Groom was euthanized following the Breeders' Cup
In the world of thoroughbreds, the horses have been inbred so much, and for so long, that genetic problems are becoming commonplace.
If you listen to the "horse people", their animals are the LEAST inbred of all domestic horses, and the problem is with everybody from the trainers to the jockeys to the guys that maintain the track.
I find it odd that you are not having these deaths with all of the other horse sports, most of which were just listed above, some of which put similar strain on both horse and rider, although perhaps not at those speeds.