[Skunkleton]

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.

[nneonneo]

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.