I don't think it is a liability at all. QFF is a separate division of the company (in fact, the most profitable). Most people never accumulate enough points for an F ticket. After all they have several million FF's but most don't really fly that frequently.
The points you see sitting in your account are actually cash sitting in the bank for QFF. Earning interest. So by definition, that's an asset not a liability.
When someone uses their points, the money goes from QFF to the supplier of the product. If that's a QF seat, it goes to QF to pay for the seat. 8,000 points for a MEL-SYD seat probably earned them between $80-120 back when the credit card co, or woolworths, or whoever bought the points, and then add taxes on top and they've easily made as much profit as a red-e-deal, if not more.
Then the really profitable part is the FF Store. The redemption value is significantly less than the amount they earned for the points, and things like the new auto-reward gift card will get those who don't earn much to convert them quickly and really make a profit. For those gift cards they're
About 3,800 points for a $20 card (even if those points are bought at 1 cent each, that's an $18 profit on a $20 gift card!)
The reality is, the points are earning them money both when they are stockpiled and when they are redeemed. And if they eventually expire for some people, that's just pure profit.