to be fair those change fees only apply to the discount economy fares - you can pay more to have schedule flexibility with a flexible/refundable fair

I've never heard a rational justification for overbooking except that it allows airlines to sell more tickets than they have seats. On most airlines non-refundable fares have a change fee which cover the fuel costs in the event of a no-show. Southwest is the only one that does it right- their refundable fares have enough margin built in to cover no-show costs without change fees.