<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by xyzzy:
Time trouble and expense have nothing to do with this. They charge because they can and because some people will pay it. They also often open up seats at the last minute that will not sell and otherwise not be full. It's all about separating you from your money.</font>
That's about the best explanation that I've seen so far. "In the olden days" prior to electronic ticketing, it made sense to charge extra for short-notice reservations. Not only did the back office have to issue the paper tickets in a timely manner, but they had to be sent out express mail.
When electronic ticketing came on the scene, many carriers dropped the short-notice booking fees for e-tickets as a way to encourage passengers to use e-tickets. For some reason, AA kept the fee. It's probably no coincidence that AAdvantage is considered to be one of the most profitable mileage programs out there. They continued to charge because of the perceived value of last-minute booking. Many passengers just accept the fact that there is a rush fee of some sort. In sort, they do so because they can.
A couple of months ago, when many carriers were changing their fee structure, several added an AA-like expidite fee even for e-tickets. Again, just because they can, and because they need the money. It's really not any more complicated than that.