It's been a while since most airlines started charging fees for checked baggage. The first time I flew since this started, I had forgotten about it and was a bit surprised when the kiosk prompted me for an extra $15. I had become used to the idea of checking luggage, since 9/11, as the security checks made it impossible to carry even a tube of toothpaste. The obvious response to that issue was, "just check it" and avoid the hassle of dragging bags through the airport in the first place. I think everyone else had the same idea, as I hardly remember seeing people with carry-on luggage.
Now things have changed. The Financial Crisis has caused many companies, including the airlines, to squeeze every dollar they can out of anywhere they can. The bag check fees are one way to do it, and like many people my initial reaction was "of course, they have to get as much money out of you as they can." But then I got on the plane, looked around, and realized that's only part of the story.