I live in Horse country. You're almost required to carry a gun and ride a horse here or they might tar and feather you , but the horse people here live here and that's the difference. They don't make a mess of their own back yards, and they are major contributors to trail building, :)and this is volcano country so the ground is pretty porous and water stays in the streams ans trails are pretty hard underneath. I 4wd and the hikers, horses, and 4wders share the some trails to a certain extent. The forest service does have horse trails, bike trails, ski trails, snowmobile trails and dog trails. Horses and dogs don't mix well nor do dogs and skiers and snowmobiles, its a safety thing to a large extent.

However it seems that horse people going into the Sierras see it a chance to take a gas lantern that lights up an entire valley and since they have gallon jugs of everything, they seem to think that they should leave the half empties behind, I guess for the poor backpackers to use. Only packers can trash an area that badly because a backpacker couldn't carry in (or out) that much stuff.

When I spent a week below Supai in the Grand Canyon - come Sunday the horse packers left and they didn't want the horses to have carry all that stuff up the trail so they would leave it all in nice neat piles in the campground. You just went down and grabbed few ammo cans of food for the next week and buried it up in the rocks.
Jim