As of 2014, there are approximately 320 million people in the USA.  If there are 40 million mountain bikers, then that is one of every eight persons in the country.  Difficult to believe.  If you have ever ridden off trail then you would realize it takes a high level of bike handling skill.  The law was written in 1964 before mountain bikes were created.  Mountain bikes do much less damage than horses and do not leave a mess on the trail.  Many of the trails in the area are maintained by bike groups.  The wilderness should be open to all modes of human powered transportation.