I agree with everyone else. Those animals are bred for it and they are very well cared for. Bucking is in their bloodlines and they enjoy it just like a good cutting horse loves cutting. The flank strap encourages a bigger buck but nothing can make
a horse buck if it doesn't want to.
Do animals get hurt occasionally? Sure. But, I have seen more horses get hurt in the cross country portion of eventing competitions (I have seen all of maybe a dozen of those on TV), than I have seen any animal get hurt at all the rodeos I have attended my entire life, and I usually go to 2-4 per year. Jumping, XC, cutting, roping, reining, dressage, heck even just loping across a pasture on their own holds risk for horses.