Just shows what a nonsense some travel pricing is.
I can understand that airlines don't like people noticing their silly pricing. I also think that it's acceptable that your return is cancelled if you skip the outbound, you are buying a consecutive service. Though if you fly the outbound and skip the return, I cannot see that the airline can really punish you for it (e.g. charging a fee or blacklisting you). It would seem to me that you should be entitled to use/consume less of, or stop early, the service you have paid a flat rate for, without penalty. I would think if the airlines did punish people in any significant way they would get sued and/or new legislation would come in and they'd either have to stop punishment or "correct" their pricing models.
There can often be related scenarios on UK train tickets. Two examples I have seen & used:
1) You want to travel from A to B on a weekday morning. A to B is classed as a peak time ticket. Yet A to C stopping at B on the same train is classed as an off peak ticket (price ~GBP 200 versus GBP 90). So buy the ticket to C, get off at B. With standard UK train tickets you are entitled to break up your journey and/or abandon your journey early, so perfectly valid.
2) You want to travel from A to C return. A to C stopping at B return ticket costs X. But A to B return and B to C return tickets combined cost less than X (I have used to save ~10%, on some routes it can save > 50%). Perfectly valid as long as trains used stop at B. This is known as a "split" ticket, reasonably well known.
