You might have people start to do things on the trail OTHER then ride. (Graffiti...) There are also a lot of dangerous things that could happen if a rider were to ride their horse at an uncontrollable pace. There is also the problem of litter, and campfires, which could start a fire. A few of these things depend on whether it is going to be a trail open to everyone or not.
I don't know all the legal terms for this, but these are some problems that you might need to prevent.