Below is the text right out of the Land and Water Conservation Fund Act. There must be some new modifying legislation because it specifically states that the Golden Age pass is $10.
I also would like to point out that in 1966 the National Parks bicentennial was a huge deal and the government spent millions to upgrade the infrastructure before the big day. In fact a lot of the existing infrastructure still being used to this day came from that spending package. There wasn't any push by the government for the centennial...
16 U.S.C 460l–6a (4)(.a.)
(.b.) RECREATION USE FEES.—Each Federal agency developing, administering, providing or furnishing at Federal expense, specialized outdoor recreation sites, facilities, equipment, or services shall, in accordance with this subsection and subsection (d) of this section, provide for the collection of daily recreation use fees at the place of use or any reasonably convenient location: Provided, That in no event shall there be a charge by any such agency for the use, either singly or in any combination, of drinking water, wayside exhibits, roads, overlook sites, visitors’ centers, scenic drives, or toilet facilities, nor shall there be any such charge solely for the use of picnic tables: Provided, That in no event shall there be a charge for the use of any campground not having a majority of the following: tent or trailer spaces, picnic tables, drinking water, access road, refuse containers, toilet facilities, personal collection of the fee by an employee or agent of the Federal agency operating the facility, reasonable visitor protection, and simple devices for containing a campfire (where campfires are permitted). For the purposes of this subsection, the term ‘‘specialized outdoor recreation sites’’ includes, but is not limited to, campgrounds, swimming sites, boat launch facilities, and managed parking lots. Any Golden Age Passport permittee, or permittee under paragraph (5) of subsection (a) of this section, shall be entitled upon presentation of such permit to utilize such special recreation facilities at a rate of 50 per centum of the established use fee