Some people are a little quick to put horses down.
I knew someone pretty well when I was a kid and she had a horse that she used to lend to my friend to ride. The horse has "allergies" of some sort which could cause a few seasonal problems, but when fed a careful diet and a few affordable, easy supplements the horse was totally normal, she went through a year with the feeding and there were no ill effects. Then, come the new season the owner just puts the horse down out of the blue and me and my friend were very shocked. The owner was very upset but said the allergies caused too much of a problem. I did not understand this one bit. About a year on her horse that she loved and rode had some foot problems, don't remember exactly what it was but again it was a manageable, although slightly more expensive (but not prohibitively so) condition. Then she put the horse down and she was really devastated. I still cannot comprehend her, she had solutions, she could easily manage and afford the solutions, but it was like she one day decided it was, just not worth it, didn't matter, and so just put the horse down. I know people would have taken those horses and cared for them.
At the same time though, its only our personal attachment to horses that makes us feel this way. You wouldn't feel shocked about someone putting down a cow that had produced many calves, horses are just animals, like cows, sheep, dogs etc, we, as "horse lovers" just put more value on their lives. And maybe this woman didn't. Many people are fine with the deaths of cows, sheep, pigs, dogs even, etc. but mention horses and people go a little insane.
There are likely to be extenuating circumstances and even if their isn't, you shouldn't dwell on this experience, you can't change it, accept it and just take precautions for your own horses, ie. If you ever need to sell encourage the new owner to contact you if they ever can't keep the horse or something.