If you can't keep your pet safe you are irresponsible and shouldn't own it. One of the things you have to keep it safe from is other humans. I wouldn't leave a dog unattended in a publicly accessible area as I am a responsible human. Many horse owners in this thread seem to think that this doesn't apply to them.
Not you obviously, you are an responsible owner who is able to keep their horse safe. I have no idea why you would encourage the ones who don't.
How do you expect them to do otherwise?
Say I've got a horse in a field with a footpath running alongside it, that many people go past every day. I put signs up all along the fence, very clear big signs saying don't feed the horses, they are on a diet, feeding them could kill them. I also use electric fencing on the outside, and put another fence further back from the footpath also electrified so people can't reach them. The gate is padlocked and also not accessible by the horses as the fence stops them. Don't forget the signs all the way along explaining the fences are electrified. Don't want a lawsuit on that.
And yet people still throw food to them, or climb the fences to get in the field, potentially getting an electric shock at the same time.
Other than standing by the field all day and shooting trespassers, what more do you expect from people? Livery yards are being shut down more and more and fields are getting built on for more bloody housing. There are many places getting taken away from us to keep our horses.
People should be able to read a sign and follow it. I know, I am expecting too much really, why should an adult be able to read and understand a simple instruction? Maybe we need fences beside roads and railways now to stop these people from wandering across them. Oh I know, we should give every adult a 'guide', someone to hold their hand all day and explain simple concepts to them, remind them to breathe, that sort of thing.