Airlines see themselves as selling transportation between A and C at a certain price. That a connecting itinerary stops at B is immaterial.
A passenger believes they are buying a seat on a flight from A to B and then on to C. So it’s ok to use only the seat from A to B, and not use the seat they’ve paid in full for from B to C.
But the airline thinks travel between A to B is a totally different product with a different price.