It's all about wanting an agreement. Yes, citing eroding earthworks by walking on them or using the Devil's seat could be used to say no agreement can be drafted tobe clear and consistent but of course those things are accepted by common consent whereas climbing DD is not accepted by common consent so the "don't offend the feelings of others" clause nicely sorts things out.
If you know the vast majority of people would not want you to, don't do it. What could be clearer, more democratic or more workable than that?
Isn't that how we all live our lives anyway, or ought to?