There's been similar things on Dover Calais ferries and Le Shuttle. Especially when the Channel tunnel first opened, it was way cheaper to buy 2 day returns (@ about £15 each) than a period return (@about £50), but they soon brought in the condition about using all sectors, and blocked UK computers buying tickets on the French site.
The rail ticket fiasco of Darlington vs Newcastle (actually it was Durham, where for some reason it's nearly always cheaper) is easily overcome now that they have auto-barriers, by buying a single ticket from Northallerton to Darlington, and the combined fare London-Durham plus Northallerton to Darlington is often less than London Darlington ! It works because in many instances, unlike air lines, 2 singles are usually cheaper than a return.