About the only thing I can possibly see is that airlines can get into significant trouble with the security services and airports if passengers who are supposed to be on the plane, aren’t.
Other than that, I suppose you’re getting into mystical areas of “we were unable to sell that seat at the staggering price we would otherwise allegedly have been able to obtain for it. You have, deliberately and with malice, undermined our capitalist right and duty to screw every last penny we can out of travelers”.
To quibble a little - the airline is not saying that you can’t change your mind. They are saying that if you change your mind and the journey you actually take would have cost more than the journey you booked, they are entitled to charge you additional money - because you booked a ticket which specifically does not allow you to change your journey details without paying extra.
Their terms do specifically state that if you decide you don’t need any unused travel coupon (i.e. your ticket), then if you booked a refundable ticket, you can cancel the unused portion and get your money back.
So from Lufthansa’s point of view, if this guy wanted to do this, he should have booked a refundable return ticket, then cancelled the last leg, got his refund and booked the Frankfurt-Berlin ticket.
They would have had no problem with that but he would have had to pay significantly more for the refundable ticket in the first place.
Most people of course don’t book refundable tickets because airlines charge you a fortune for the privilege.
It’s amazing how the use of technology to find and maximise price efficiencies is apparently only supposed to work in their favour.
¯_(ツ)_/¯
