I think that they are meant to run free on a bunch of land and live on grass like they do in the wild. Not living in a stall just standing there. Horses are meant to eat all day because their stomachs don't stop digesting. And living in a stall won't allow them to do that.
I think that animals are changing. Just because there are horses in the wild that run free all day, doesn't mean that domestic horses should also do the same. Being in stalls also doesn't mean just sitting there doing nothing. People do give horses hay and water inside of the stalls. So, the whole food thing isn't the problem. Now, don't get me wrong, I don't think that horses should be in their stalls constantly, but they can be in them some of the time. For example, they should be inside at night. In the wild, they are in a herd, so there are many other horses to protect each other, but in captivity, there is usually only 2-3 horses together at a time, unless it is at some huge stable somewhere. Even at a huge stable though, the horses are domestic, they don't know how to protect themselves as well as a wild horse, which is most commonly extremely more, well, wild.