FF miles are, as Rudi correctly pointed out, a liability. However unlike the figure Rudi mentioned, airlines only value them on their balance sheet as 40 cents/1000 miles or 0.04 cents / mile.

This topic was discussed earlier this year on FlyerTalk. You should go back to the subject:

Mileage Redemption Tips by Michael Greenberg NBC
started by pgupta on 8/6/98 for more info

Although airlines do have somewhat of a liability - ff miles have absolutely no cash value - also airlines impose capacity controls - so ff awards *are avoidable* for airlines - they just have to deny awards which they can legally do (of course, at the expense of customer goodwill). My main point being is that airlines are not compelled to have award seat availability that corresponds with mileage being handed out.
I think someone already pointed out that if airlines allowed unlimited award seat redemption to Hawaii - major airlines would need extra flights just to accomodate all the requests - hence the point that are not bound to provide seats merely because they issued ff miles and placed a destination on an award chart.