I know, it hurts my brain too, but check this Wired post: Dropping oil prices hurt airlines. Huh? It's what happens when airlines literally hedge their bets on oil futures. From Wired:
But let's turn that scenario on its head and say the airline hedges at $110 but the price drops to $92. Oops. Now the airline is paying more for fuel than it costs on the open market, placing it at a competitive disadvantage. The balance sheet craters.
This is why I am not a gambler--my luck tends to be more like United's than Southwest's.