If I pay the airline to fly A-B-C, but only fly A-B, I've still paid for the B-C segment. I've also let them fly that segment with less fuel consumption due to lower weight. Really I don't care what price they've evaluated that segment at, if I pay for it and choose not to use it, I've still paid for it.
For me its like buying a combo at McDonalds, its often cheaper to get a Big Mac combo, then to buy a Big Mac, Fries, and a Drink seperately. Therefore I will take the combo and throw out the fries. I still paid for the fries, so McDonalds can't punish me for throwing them in the garbage.
Its not my fault McDonalds chooses to the give me a discount for taking the fries over not taking them.