If you're so concerned about humans and corporations ruining creation (a valid concern), why let people into the National Parks to begin with? It's not like a privatized camp ground is going to be any cleaner than the current ones.
Because I don't want to see hotels and amusement parks or attractions popping up around throughout these areas, and I sure as hell don't want these lands being sold off for mining and drilling. How much leeway are corporations (which obviously pollute far more than your family of four camping out in the mountains) going to have assuming these lands are, in fact, privatized?