Basically, that's what it is. The park or local government is selling these "passes" which really amount to insurance, so that that government has the money to pay for rescues if they need to (or if rescuees don't/can't pay), and maybe also for other expenses involved in park upkeep. "Use fees" for public parks are not uncommon. Virginia State parks generally charge such fees for parking, for instance, as does Grand Canyon NP.