What do they do with all those empty seats when the pax throw the unused
return ticket in the rubbish bin?.
You may find that most of the seats
aren't empty, because from experience the airline revenue managers know they'll have a higher no-show rate on the return leg of cheap return tickets, so they set the overbooking level correspondingly higher...
Oh, and incidentally, return fares being much cheaper than single fares is neither new nor unique to KLM - carriers have been doing this for decades.