@Moonraker 2 because the NT is imho a grey area between being a disguised big business rather than what most would consider as a Charity, so it probably makes better business sense to do it like this.
I can’t imagine them ever opening up any of the trusts premises car parks for overnighting, their often obnoxiously over enthusiastic volunteers that police the places like it’s a vocation would be mortified for starters.
Plus there’s probably all sorts of caveats in the arrangements they have with the trustees and often true owners of many places that restrict what can be done. Powerful as the NT is they probably can’t get away with everything they would like to do even if it brings in more money to build more visitors centre selling imported crap from china.
They would have to recruit more volunteers to act as rangers following their unwelcome guests around with litter pickers and doggy bags.
You would probably get a knock on the door every few hours to remind you of the rules and confirm you are self contained, the latter is already happening at the RSPB location in loch garten since the honourable idea of allowing a max of five vans a night backfired on them
I think the NT sites are best left as they are, often extremely nice but expensive visitor attractions with handy free carparks (for members) we certainly couldn’t afford to visit more than a couple a year if it wasn’t for being members