I could probably be convinced of this reason.
But why would they cancel B-A when there’s a no show for A-B? More so when there’s a few days gap between A-B and B-A? The only issue being they were booked as a single itinerary/PNR. I don’t see what cost has got anything to do with it.
Because they could use the now "vacated" seats for:
- Last minute travellers (who pay significantly higher for this)
- move their own personnel from B to A
- alleviating problems caused by overbooking, canceled flights, delayed flights or any other disruption.