> This is insane on a whole other level, because United profits when you don't get on that 2nd leg.

Not necessarily. If you use use hidden-city ticketing, they lost out on extracting the extra money they wanted to charge you for going to your intended destination AND the extra money they may have been able to charge someone else for the leg you didn't take.

That said, I think it's the airline's fault for using an over-complicated pricing model with odd behaviors that customers can take advantage of like this. There'd be no issue if they just charged the same per leg regardless of your final destination. That'd make everything transparent and totally remove the need for hidden-city ticketing.

It depends how you look at it. Once you've booked your ticket, United is better off if you don't take the last leg of the flight than if you do. Prior to booking, they would of course prefer you book the more expensive ticket. Forcing you to take the last leg is about encouraging you to book that more expensive ticket; it's not because they actually lost something when you chose to skip the final leg of the journey rather than take it.

> they lost out on the extra money they may have been able to charge someone else

Not necessarily. Airlines have sophisticated yield management programs that should give them a good estimate on how many passengers are likely to skip the final leg of any particular route -- the airline can oversell the final leg by exactly the number of passengers who aren't likely to show up.

and am incorrect in thinking that if they now have an empty seat because you bailed on the leg, that they can let a standby passenger fill the seat?

I think they will just carry less fuel. Hauling around fuel - that has weight - you don’t use will just result in you burning more fuel than necessary.

Heck, jetliners actually can’t land if the tank is too full - https://youtu.be/R9oqi6HteJg