United Airlines Threatens to Engage Collections for Passengers Who Skip Segments (viewfromthewing.boardingarea.com)

Thanks for that - appreciate the response.

Is it a tax on the person, collected by the airline, or a tax on the airline, charged at a per person rate?

Contracts have to be argued in court, but you're going to have a tough time arguing that case on a contract whose terms you intended to breach even as you agreed to them.