Not only do I think the horse poo is an annoyance, but I beleive it could actually spread disease. Think about a hot summer day. When the trails are dry and that horse poo has dried and turned to dust. Hikers/bikers have to inhale all those particles as we use the trail. This could potentially lead to respiratory problems and other illnesses.
I don't know, maybe I'm blowing this out of proportion...
I doubt that would be a concern or horse dung would not be used as a manure/fertilizer for food crops!