The problem being alluded to above is that if you smash your head into a rock and turn yourself into a vegetable it can affect everyone else. You may be ok with the danger, but your family who now has to care for you for the rest of your life might feel that the Park was unsafe, so they decide to sue. The State gets sued and decides this is is no longer worth the benefit to others. Let's just shut down the mountain biking so we no longer have to deal with these lawsuits.
I defer to my original post on this subject.