The horses and bulls are bred carefully, as I just said. Conformation, skills, disposition, ect, are all considered, just as if you were breeding saddle horses.
And as for being treated well, I don't think I know a horse that wouldn't love the life of a bronc. What's so bad about being trailered around for while, bucking for a few seconds, run around the arena and putting on a show and hang out in the back pens and eating for the rest of the day until it's time to go to the next place?
And the rest of the rodeo stock (calves, steers and bulls) they live a pretty good life too. Sure, some get hurt, but they just get sent to the feedlot and get eaten like any other cow, or if it's too bad for that, they'd get shot like any cow would on a ranch.
The bulls, in particular, get treated as good as the horses. They're pretty spoiled.
I'm sure you like your steak and leather saddles, so you just gotta face it and understand that raising cattle ain't sunshine and lollipops, and rodeo stock kind of get a "Get out of Jail Free" card XD