Are you suggesting with your comment about "open land" that this is a bit of a con and is really only for the "Visitor Centre" and "Car Parking" so that, if you want you can go just onto the main site without paying if you are prepared to forego these 2 "benefits"!! Wiki does say that the site is "owned and run" by the NT rather than being "common land"
I couldn't really find a good term for it, but all the sites I put down as 'Open land' were ones that you could just walk up to.
When I visited the Giant's Causeway (2007) it was free to walk up to, the area was run by the National Trust who had a cafe/ gift shop there, but no charge to see the actual site. I guess it would be pretty easy to charge a fee though as access is pretty limited due to the site being inaccessible from 3 sides due to the sea and cliffs.