Buying returns which are cheaper than singles is a common practice. Part of the "overbooking" calculation takes this into account, so the carrier will know that a certain percentage of those on cheaper tickets never come back, and they take additional reservations to maximise their own revenue.
BA now offer both "cheapest fare" single and return pricing on their website. A while ago a return from London to St Petersburg, Russia, was £280, and the cheapest single (which is what I really wanted, as we were returning by car) on exactly the same outward flight and in the same class, was £1,200. If carriers wanted people to relate their bookings to their actual requirements, they would stop nonsenses like this.
"Yes, sir, fixed price menu for lunch, £15. But you didn't eat your pudding, you bad boy. In that case the bill is £45 please."