It's not just Stonehenge - the National Trust do it for most of their properties too, but they don't actively police people selling the photos
Apparently as it's their property, any photos taken on their property (if it's taken from public land it's okay) are owned by them under terms and conditions that don't seem to be visible anywhere and by taking photos you're agreeing to them.:(
Slashdot Mirror
It's not just Stonehenge - the National Trust do it for most of their properties too, but they don't actively police people selling the photos Apparently as it's their property, any photos taken on their property (if it's taken from public land it's okay) are owned by them under terms and conditions that don't seem to be visible anywhere and by taking photos you're agreeing to them. :(