I would be skeptical as well. Who owns the land? Who is allowed to ride there? If it's state/federal land, who would be sued if there was an accident? I know you said legal public trail, so I don't understand what that person means. The only thing I could see happening is that whatever you build being taken down. That is if you're building bridges/jumps/etc. If it is just difficult terrain, someone could always walk through/around it.