Continental and AeroMexico have threatned Travel agents that if passengers do not use the return portion of their tickets, they will treat the ticket as a one way fare and charge the issuing agency with the difference. Can you imagine buying a value meal from Mcdonald's and after eating the hamburger deciding not to eat the Fries. If Mcdonald operated like the airlines they would say since you did not eat the full meal you are not entitiled to the value discount so you would owe them the differnce between what you paid and what the ala cart items add up to.