[azstar]

I'm not defending the airlines smorgasbord of fees, but I can tell you why they do it from the airline perspective.

The vast majority of seats are sold at what the airline considers "deeply discounted". There are a limited number of seats sold for each flight with certain conditions, such as advance purchase, required to obtain the discounted price.

Let's say you wanted to take a 10AM flight, but it was much cheaper to take the 3PM flight. That's why you bought it. Now, you show up early and you think you're doing the airline a favor by taking the earlier flight, since they will probably have seats available anyway.

The airline doesn't see it that way. They see it as you getting on a flight that should have cost you more, and leaving another vacant seat on the flight you originally paid a lower price to take. A ticket is a "contract of carriage" which the airline claims you breached if you switch flights.

I'm not saying I agree, but that's their point of view.