Lighten your load, use more fuel?

Q. I recently flew USAirways and they charged $15 for my checked bag. I noticed a number of passengers bypassing the charge and bringing large suitcases onboard as carry-ons, which quickly filled up the overhead storage. By the time I made to my seat, all the overhead bins were full and I had to check my carry-on. Now, I thought the reason for these checked bag fees was to make up for fuel costs. Doesn't allowing all these bulky suitcases as carry-ons sort of defeat the purpose?

A. Not quite. Passengers who avoid the checked luggage fees must pack less and adhere to size/weight limitations. Lighter planes use less fuel, so it all works out.