This is just an oversimplification though. If you had any experience about travel industry (or logistics) you would understand things much better.
Here is an example for you (from logistics): Sending a truck from Berlin to - say - Györ may cost 3 times less than sending the same truck from Györ to Berlin - even on the same exact date.
Is this because shipping companies try to make money out of nothing, for you?
A fair comparison would not be the return, but Berlin-Györ being more expensive than Vilnius-Berlin-Györ. Is that common in logistics, in your experience?
This was a fabricated example, actually: I work in tourism not in logistic (but I have friends in that field).
My point was that to the layman this does not make any sense while if you are managing a shipping company you soon realize that some destination are more profitable because your truck that was maybe taking specialized replacements parts from A to B can easily pick up some other stuff to send back to A, while travelling in the opposite direction your truck has a high chance to travel empty on retutning to base... but you still have to pay the drivers, the fuel, the maintenance and possibly tolls.
Airlines tend to work like this: at the start of the "season" analyst define the margin for each seatbof each departure. Let's say that for Sunday flight from Venice to Hamburg, Economy, your target is 112€.
(For simplicity I will discuss only a direct flight, but the same idea applies to any leg of any Itinerary).
Yield manager for that area/period has now the task to make sure he gets 112 or more on each ticket. And take in account that an unsold seat gets 0, so lowers the averge margin (which is what the yield is calculated upon).
This will soon make you realize that any chance to sell again a newly vacated seat is a boon.
So in the case of the OP the company can either assume that it was a honest mistake and he will somehow miracously get there in time to get on the second flight (3% chance?) or assume that he decided he does not care anymore, he had a serious accident, got fired, won the lottery, whatever (97%) and promptly put the seat back on sale.
The problem with a-b-c costing less than a-b is less obvious, maybe, but it has similar causes: for the airline it is more efficient to sell you the itinerary with a stopover so their pricing reflects that.
There have even been attempts to take passengers to court for getting off at the intermediate stop (they were dismissed) so it's definitely not just because the airlines are throwing a fit if you decide to change your plans.
Probably the a-b leg and the b-c legs sold alone are not very popular so they want more money to maximize the yield, while everyone wants to go a-c and return.
It seems to me that since airlines can't force you on a plane except for taking your luggage hostage, you're free to drop as long of a 'tail' as you wish. I'm wondering whether they'd put you on a black list or something for doing this consistently.
The reason is happens is that take for instance ATL (former home). ATL is a Delta hub and has direct flights to a lot of places that other airlines don’t. Between people preferring direct flights and the lack of competition, they can charge more.
But flying out of MCO with a layover in ATl, they lose the non stop flight advantage and they have to compete with other airlines.
Also ATL sees a lot more price insensitive business travelers than MCO. Businesses aren’t going to force their salespeople and consultants on one of the low cost carriers.
The full explanation would take a wall of text (and still let you unconvinced because you feel entitled to do as you please, probably).
Super-condensed version: civilian flight are a pretty difficult "product" to handle efficiently. Price increases until 1 minute before closing the airplane doors, then falls to zero.
On top of that, the product "provider" also needs its own product in order to move personnel and technicians all over the globe, but of course they cannot just cannibalize their own products beyond the point of profitability.
Plus they have to handle rebookings and passenger protection in cases like delays, sudden airport close-down and so on. (Have you ever been on a waiting list, btw?).
All this is pretty complicated to manage already, so they need to exert as much control as possible on yield and occupancy.
TL;DR: a flight is not a bus ride. So if you just decide to cut it short the airline will try to reuse your vacant space for whatever reason.
Yes, it is not exactly the same thing but the point is: by getting off at B you are making the B->C flight travel with a wasted (empty) seat.
Which they would have preferred to either sell to someone else or use for moving a pilot or technician to C.
(Note also that this trick of getting out mid-itinerary only works if you do not have checked baggage, because that will arrive in C, and neither the airline nor the airport will be happy to reroute it to wherever you thing you want to go next.
Flying is expensive and logistically complex. Just making sure you end up where your ticket say is complicated. If you (as a customer) decide to change your plans you are making everything more complicated (and possibly preventing other customers to pay for the whole itinerary).
The actual question here is why they won't sell you a ticket for A->B for the actual cost of that leg of the A->B->C flight, and then sell the same seat for B->C to someone else.
See above: let's say that on Frankfurt-Hannover and back you get on average 52 passengers a day, and Hannover-London is more or less the same, while people flying Frankfurt-Heathrow are so numerous that direct flights are always full and therefore you need to offer FRA-HEA with a stopover to satisfy the extra request.
If you are sure that 80% of your passengers will go to Hannover only to then fly to London (and back) your prices will reflect that... and Frankfurt-Hannover cannot be lowered too much because you still has to try to reach your quota for the flight per se.
Of course I can understand it from their point of view. But this doesn't make it any more sensible to me as a consumer of their services.
In the aforementioned situation I wasn't trying to exploit the airline, it was a simple mistake that happened and could be easily alleviated. But the rigid processes, precisely the ones where accountability sinks, made it impossible for the humans involved to correct the mistake.
I still stand by the ridiculousness of that. If not the logistics quirks per se, then the fact that this completely unrelated matter dictated the resolution of the situation against common sense and my interest.
What makes this even worse is that presumably the PR department of that very company had to be involved later and they still spent their employees' time and money to compensate me for the mistake that could be corrected for free.
Here is an example for you (from logistics): Sending a truck from Berlin to - say - Györ may cost 3 times less than sending the same truck from Györ to Berlin - even on the same exact date.
Is this because shipping companies try to make money out of nothing, for you?