Are you familiar with horse slaughter?
Horses are prey animals with an extreme sense of danger. Unlike when cattle are led to slaughter, the horses instinct of danger kicks in and they basically go wild in the shoots and are hard to get a clean kill shot in the head. Many are injured during the process of leading them in. This is what the animal rights activists use to portray the cruelty of horse slaughter.
You are correct that many people buy horses as you described and some release of them in the wild. Others simply cannot afford to feed them and they also release them. This can create huge ecological problems as horses are designed to eat almost constantly and they will eat the wild grasses down to the ground and then pull the roots out and eat them too. That is why you typically see horses in dirt pens because they have eaten all the ground cover and in relatively short time there is no more grasses to forage so they move on.
Until people realize that horses need to be slaughtered as humanely as possible, this cycle will continue.
What happened here was nothing more than evil and no animal should be hunted just for the pleasure of seeing it done.
And yes, I do raise and breed horses as they are truly a magnificent animal if trained properly. But they are also a huge pain in the ass, as a bored horse can be quite destructive and even harmful to themselves.