The point you’re missing is the horse is often left alone in a field in all weathers. Often a bare muddy field with hardly any grass. To a non horsey person the horse might look neglected, lonely and hungry.
To be honest not many horses are 'left alone' because they're herd animals and the majority of horse owners understand this, and therefore they usually have at least one companion, if not a herd. They're on a muddy field with barely any grass because a) horses weren't designed to live inside, they are designed to live outside, they can adapt to living inside due to saving grazing land being poached (horses poach land when it gets wet, leading to mud) and they're normally obviously within the boundaries of a farm, not just a random field in the middle of nowhere.
Secondly rolling in mud, and foraging for food are perfectly natural behaviour for a horse. My horses field is muddy as hell right now, they get a haynet each twice a day, they are leaving a lot of the second one because they'd rather be eating the grass. Randoms don't know I'm feeding them haynets twice a day, do I now have to stick my feeding schedule to the gate too? And anyway, what would that mean to people who are self professed to know nothing about horses? Why aren't people campaigning about wild ponies on Shetland, or on the Welsh mountains, on Dartmoor and Exmoor if they're so concerned? How on earth do they think wild horses survive? It's not about worrying about their welfare at all because the correct course of action if you are concerned is to report to the correct authority. Not take it upon yourself to feed an animal you 'think' is hungry with things you have no Idea of the effect they're going to have.
It’s not like feeding a horse when the owner is riding it, or feeding a dog in the park.
No it's like feeding a dog that's out in the garden, how many people would take it upon themselves to do that and then argue that it's ok because it was alone and 'looks hungry'?
Yes it’s wrong to feed horses and ponies... but calling people thick for not knowing a handful of grass can be harmful; that sounds rude and arrogant. And not a great way to keep your horses safe.
You don't need to know that a handful of grass can be harmful. Many have explained over and over that it is on here, but people are still arguing back about it. All you need to do is not feed an animal that's not yours, without permission. I own the horse, I put the time in to learn what is best to feed that horse, I pay the bills, I'm the one that gets to decide if 100 carrots and a big Mac or a haynet are suitable for my horse to eat, not people who freely admit they know nothing about horses.