People are cross because they've had vets bills, dead horses or hours/days/weeks watching an animal suffer and usually because people won't listen, or assume the horse owner is being precious, are rich and above themselves and so want to show them by feeding the horse anyway, with absolutely no knowledge about a horse's biology or digestive system, just that the horse likes an apple, or a polo or a sugar lump and the person likes the feeling of giving it to them, so it must be all good.
I caught someone trying to feed my horse a pineapple once, and got told to fuck off because I'm a posh cunt when I asked them not to. I've also had people trying to sit their kids on my elderly, retired horse who was patient and kind, but having someone sat on him was uncomfortable and hurt, even a small child, confirmed by vet diagnosis and x-rays, he was living out his days in as much luxury as I could provide him, just being a horse, but someone with no knowledge or permission did that to him, having scaled 2 fences laden with private property signs, and then told me how awful I am for advocating for him by telling them to stop, not to mention the threats of burning the farm down. Would you not be angry and frustrated at that happening repeatedly?
If you're truly interested then think about the areas wild horses still inhabit, I'm not being sarcastic here, but are there any polo or sugar lump trees in the wilds of America where herds of wild Mustangs live?
It's a harsh environment, as is just about anywhere a horse has developed and thrived, Shetland ponies or Welsh Mountain ponies, Dartmoor, Exmoor, for example - sparse land that requires hours of roaming to provide the same level of sugar in a day that a packet of polos or a bag of apples contain, and there's competition for high value sugar laden foods like apples or anything like that that grows naturally, for a start there's 30-40 horses in one herd eating it, rather than 5 or 6 in a field, there's other animals competing for it and it's only available for a short amount of time, so in order to get enough to see them through winter to spring when there's more food, they will eat whatever they can get hold of - because it's rare, they'll still do that when presented with a high value food source, even though they (should) be getting everything they need from the person looking after them, because that's how they've evolved and survived.
Horses getting too much sugar is the other side to the coin of not getting enough, both are bad and lead to problems for the horse, and it's a delicate balance.
And that's just the nutritional side of things, horses have digestive systems that have evolved to eat small amounts almost constantly, large amounts or bulky food causes problems because it messes with that, they can choke and horses can't be sick in the same way other mammals can to rid themselves of excess food, or something that they can't digest or is making them ill, so it clogs the gut up or is absorbed where is goes on to cause more problems.
Then there's the behaviour it causes as they compete for a food source because their evolution tells them they need it.