It's not clear why exactly the airlines are so upset about this. If the passenger isn't present they can just shoehorn a standby onto the flight, or at least save on fuel costs.
They're scared that it might become popular and cost them too much.
The airline's business model is to try to charge people from point to point, this scheme takes advantage of that fact to find savings.
The airlines could, instead, change how they charge (e.g. per land mile/per minute in the air) but then it becomes harder to justify why direct flights that are cheaper for the airline to run cost consumers more, not less. And why inefficient indirect flights cost consumers less in spite of being more expensive for the airlines.
Airlines need to keep their current business model to prop up their hubs, but they need hubs due to their current business model. If you invented airlines today (particularly with the new Airbus and Boeing aircraft coming on-tap with greater range) you'd see far fewer hubs, and more direct point to point flights.
This is airlines trying to slow the inevitable, which is that they need to evolve.
Because the person was supposed to pay more for the privilege of skipping that flight leg and just staying at that intermediate destination. That customer is robbing the airline of their god-given entitlement to extra revenue.
As already pointed out, the airlines lose no actual money, but you are right in that they lose the theoretical difference between the discriminated price and the one they paid. That's not a legal debt, though.
Because a ticket from x->y->z sometimes costs less than a ticket from x->y. Presumably this has something to do with how much the markets in x, y, and z are able to bear. As in, people in X and Y will pay more for tickets than people in Z, so tickets terminating in Z are cheaper.
It's usually simpler than that - price competition is a major factor in most of these cheaper tickets. Airlines don't think in terms of segments, they think in terms of routes (because that's ultimately what a consumer bases shopping decisions on). If a competitor has a cheap and popular X-Z flight, then you will want to make X-Z cheaper even if you don't have a direct flight. But, if your competitors don't fly X-Y, or they charge more (because they have to go X-W-Y), then you have zero incentive to make X-Y cheaper.
Airlines generally also avoid empty seats, because it almost always means lost revenue. They'd rather overbook and risk having to compensate bumped passengers than have no-shows. Combine these factors, and you can see why they're really unhappy about people skipping the Y-Z flights.
Of course, in most situations, consumers are not penalized for using or consuming only part of a purchased good or service, even if they deliberately discard part in order to save money. United is in the wrong here, and I doubt a court would agree with them.
The airline's business model is to try to charge people from point to point, this scheme takes advantage of that fact to find savings.
The airlines could, instead, change how they charge (e.g. per land mile/per minute in the air) but then it becomes harder to justify why direct flights that are cheaper for the airline to run cost consumers more, not less. And why inefficient indirect flights cost consumers less in spite of being more expensive for the airlines.
Airlines need to keep their current business model to prop up their hubs, but they need hubs due to their current business model. If you invented airlines today (particularly with the new Airbus and Boeing aircraft coming on-tap with greater range) you'd see far fewer hubs, and more direct point to point flights.
This is airlines trying to slow the inevitable, which is that they need to evolve.