It's not ironclad for Qantas though ...
I think with QF domestically, these days it's not very common to find fares from A to B to C that are cheaper than just going from A to B. They do happen, but perhaps they are not common enough (or used enough), for them to make a big fuss about, perhaps that's why "it's not ironclad."
With the hubbing system in the US, much more common there if wanting to travel to a hub, cheaper to fly onwards. Vaguely recall reading (here or more than likely on FT) that people have been charged in the US for completing their journey at an intermediate point.