It's different because you want to own a horse. A horse isn't a cow, it's a pet. You don't need it, you want it. You want it even if you can't look after it. Tough, be responsible.
Motivation for ownership is irrelevant. Horse owners don't need to justify their reasons for ownership to anyone (that's aside from the cruelty ethics surrounding steeplechase racing, with which, FWIW, I do have a problem). This thread relates to the behaviour of the public and feeding of horses which are not their own. The onus for that is on those people, not the owners.
It's not the same situation as foxes in the hen coop. The onus there is entirely on owners to protect their animals. This problem is caused by humans: supposedly sentient beings endowed with some level of skill in communication. It's hardly beyond human wit, or basic good manners, to comply with owners' requests not to feed their horses.
This thread shows an enlightening divide in the tone of the responses. People can't be blamed for what they don't know. Those unused to horses likely won't be aware horses can't vomit, or know that every horse-lover dreads colic, a relatively harmless condition in humans but frequently fatal in horses. Some posters genuinely haven't known why feeding horses grass is dangerous, have asked, been enlightened and resolved not to do it again. This is how decent people react, and every kudos to them.
It's the others, the kind of idiot who can't take even the mildest criticism, who won't be told what to do and thinks they know better, who gets on the defensive and insists they'll continue their unwanted behaviour, who are the menace to these animals' wellbeing. There are plenty on this thread who've been warned of danger to the animal and potentially to the child but are determined to continue 'because it's nice', or because they think their little darlings have the absolute right to do exactly as they please and fuck the consequences to others.
In that mindset of course it's the owners who are to blame, for daring to own the animals in the first place. Or their motivation is to blame, for owning them simply because they 'want' to. Why else take on the care of an animal; why have children? Here's betting those people don't ask the same question of those with cats, dogs or kids.
Fekinell.