I frequent the polo matches on Long Island. I know some of the players and can say firsthand that their first concern is for the horse. I have never seen anything but the best of care. The polo players I know don't just use ponies from Argentina: they use ponies from all over the place. One of my favorites is a dun appaloosa.
Polo ponies are expensive. Yes the sport is rough but few people would have the money to be continually replacing horses that are neglected. Injuries will happen with ANY sport especially one that involves the kind of contact polo does. I've described it before as hockey on horseback and one of the players agreed with me.