I really hate to wade into this mess, but I just can't take much more of the "fraud" allegation being tossed around. I'm not a lawyer, but I am quite certain that buying four legs on an airline and arranging them in the most cost effective manner does not constitute criminal fraud.

The airline controls its reservations, now more than ever since so many tickets are issued via the airline's own website and since each reservation has a unique FF number on it that easily allows the carrier to see all reservations that a given passenger is holding at one time. If the airline doesn't like what you have booked, it could easily cancel the legs and refund your money, on the spot, or perhaps even refuse to issue the offending ticket in the first place. I would maintain that by accepting your reservations, printing your boarding pass, and allowing you to fly your evil concocted tickets, it has accepted -- and blessed -- the way you arranged the segments. As technologically sophisticated as the airlines are with inventory management, don't tell me that they could not institute an automated system to immediately cancel any booking that created a non-contractual (I refuse to term it "illegal") set of itineraries.

At the very most, engaging in back to back ticketing constitutes a breach of the passenger's "contract" with the airline, and if the airline felt damaged by this it could sue the passenger for said breach. To even hint that this sort of thing rises to the level of criminal behavior is patently absurd.