In the 1980's, well meaning yet misguided officials banned bicycles from wilderness areas, yet still allow hiking and horseback riding because they have proven to have low environmental impacts. Since that time, studies have shown that responsible mountain biking lies between hiking and horseback riding in actual environmental impacts. You can read more here: http://www.americantrails.org/resources/fedland/BikeWilderness.html
Lacking any valid wilderness protection justifications for the mountain bike ban, it should be rescinded as soon as possible to allow more Americans better enjoyments of our pristine wilderness areas and increase the base of support for future designations.