Another thought that crossed my mind, since most of these people don’t live with their horses within horseback -riding distance of most of these trails, so these people usually have to go hitch up a horse trailer to a gas guzzler of a pickup truck and drive sometimes a pretty considerable distance to take their horses out. Often the parking areas at the trail heads aren’t particularly well suited to accommodating a truck and horse trailer, and they end up being kind of a parking nuisance on top of that.
And the horse people will try to retort that dog poop takes longer to break down, smells worse, and is more likely to spread disease
Which is true, still doesn’t justify not cleaning up after your horse. Just because the horse manure is “less bad” doesn’t make it ok.