Interesting that the conversation doesn't touch on whether the clause in the terms is actually legal or enforceable.
The conversation has touched on it, several times. ExXB has told us that the clause has been challenged in various courts over the years, and has ultimately been upheld as legal.
When you get on the aircraft, you don't become an indentured slave and I would suggest they airlines would have a hard time convincing a court that the passenger must be restrained and shackled to their seat to complete the journey. Thats in effect what the clause is suggesting.
Not really. The clause just gives the airline the right to claim back from the passenger the difference in price between the cheaper A-B-C fare the passenger paid and the more expensive A-B fare that the passenger flew. According to ExXB this can and does happen, especially when the ticket has been booked through an agent.