The rules are there and are explicit. As I mentioned earlier I know of a case where a corporate travel agency was sent a series of ADMs by an airline for 'throw-away' tickting of this nature and they had no choice but to pay. The airlines have more leverage over an agency than an individual, as they can cancel the agency's accreditation and even ultimately its IATA/BSP/ARC membership.
There are parallels in other forms of transport too, ridiculous as they sound. There was the recent case of a man who bought a rail ticket from London to Newcastle and decided to get off at Durham (which is a few miles short of Newcastle). I know this got publicity as the rail company tried to charge him the fare difference and penalty - I don't know what the outcome was.
I also know of a case where an individual repeatedly used only the outbound portions of return tickets and the airline cancelled his FF membership and he forfeited the miles he'd earned.
Whether an airline has succesfully sued a passenger for a difference in these cases, I don't know.