Here's the problem: national parks are supposed to belong to everyone, for all Americans to enjoy. Require a fee, and suddenly it's not "everyone's" park anymore. The higher that fee goes, the fewer people can afford it. Doesn't take long before a "national" park becomes nothing but a rich people's playground. It ceases to have meaning or relevance to anyone else, so they don't love it, so they don't protect it. How well do rich people typically protect nature? *looks at broiling planet* Not well at all. Having a few "free" days doesn't help, because then the park is monstrously overcrowded and not fun. What's the point of going if you can't see the bison over the crush of people? You'd get a better view from your television, but again, that's not likely to make people fall in love with nature. We need to get out in it, and not when there are 50,000 other people crammed in with us.
That means, if we want to support the national parks, and keep them available to everyone, we need to do it with taxpayer money. No fee to enter or use the facilities, because you ALREADY PAID FOR IT. If the parks are overcrowded, clearly we don't have enough of them and should designate more. The government already owns vast tracts of land it could designate. Sometimes it forecloses on big ranches or other private tracts that could also be turned over for public recreation and/or wildlife restoration. We need more room to plant trees to suck the carbon back out of the air anyhow. Why not kill two birds with one stone? Get people out in nature, invite them to plant furnished tree seedlings, fall in love with nature, save the world. \o/