I'm not sure I understand the distinction. A ticket to IAD includes "access to" the area outside the sterile part of the airport. You (normally) don't access it when transiting. The lack of physical access control seems more about the degree of enforcement than anything else. The faregates are only there to enforce the policy that you need the right ticket.

There are fewer practical ways to physically enforce the airline's contract. Would the HCT conversation change if your hidden city was in a different country with airside transit, and you went through immigration there?