I think neglect is terrible, and its just as bad as abuse, so I'm not trying to justify anything but to be abuse is action causing harm, where as neglect is in-action causing harm.
I don't mean to go dictionary here but abuse (in the way we use it) generally is defined as: to treat with cruelty or violence. I think an abused horse is going to have a whole different range of issues than a neglected horse, so, at least for the horse owner, its important to have these things as two separate issues.
In my experience, abused horses often have issues with people touching their heads or legs, or all over, and can grow violent out of fear, but neglect cases don't usually have this - besides them not being used to it. Neglected cases may have their range of health problems, and perhaps are shy, but these are very different from a horse that has been intentionally harmed.
Neglect is terrible though. I am of the firm belief that once you have an animal you are responsible for it until it dies or until you find it another responsible party. That means insuring that it gets all medical care and upkeep, as well as adequate protection from elements, adequate feed and nutrition, as well as water etc. I think it also means caring for the animal in a way that if you couldn't care for it others could - like you wouldn't let a horse run wild, you'd make sure you and others could handle it. I believe that it is the responsibility of this party to do all research required to fulfill their role. If they do not do this I don't think it is abuse, but neglect, but neglect should also have very harsh penalties (in my opinion).
I find it difficult to understand the actions of people sometimes, I can't imagine how people can view animals as just something inanimate, like a chair. My mother and her boyfriend live in a place in Australia that is pretty wild, its over 40 degrees (celsius) everyday and people there actually eat cats or dogs or eagles or lizards. Whatever they can catch. They were going away to Darwin for a few nights and taking the dog but leaving a young cat, barely more than a kitten, who had been kept primarily inside most of its life. My mother said that they can just jam a window out of sight open and the cat can go in and out as it pleases. Her partner refused to do this and chucked a hissy fit, saying that they had to lock the cat outside, alone because his TV might be stolen. My mum told him how given the people that populated the area, if they wanted a tv they would just break a window, and that they couldn't leave the cat alone for the first time, locked outside and scared, where other animals could take his food and he'd have no where safe. Anyway my mum ended up staying while her partner left with his cousin over Christmas. She was so upset that he would put the welfare of the TV above the cat, and not even give it a second thought.
So long and a bit OT, but I just wish I could understand why people think they way they think.