They're hardly amusement parks. Many don't have gates, but the most popular ones do need to control the flow somehow. There are roads, and trails, and sometimes

They're hardly amusement parks. Many don't have gates, but the most popular ones do need to control the flow somehow. There are roads, and trails, and sometimes lodging. People can buy souvenirs from affiliated nonprofits. Aside from that, though, there's a lot of emphasis on education, not entertainment. Also, I've experienced an order of magnitude more joy and awe in various National Parks than in any amusement park.