The term liability is just an accounting term. In the case of frequent flyer program, large / growing liability is not a bad thing, if managed well. A bit like debt. If managed well, debt are good.
At the end of the day, Qantas sold lots of points, get the cash, and will have to provide some services on those points at a future date. Qantas controls service inventory, Qantas control devaluation. The only risk is to piss off members (and kill the golden goose), or be forced by some regulator to increase inventory.
I mean, feel free to give me a lot of money now, in exchange of ill defined services I might provide you in the future at my own rate. I would love this kind of liability
