I always thought that the reason why one-ways are much more than half the return price is that the former allow customers to circumnavigate fare rules attached to return tickets, especially on minimum stays. On long-hauls, most airlines charge the Hell out of customers staying for just a few nights (and not including a Sat/Sun night) as they know that most of those are coporate travellers. I think the only reason why BA switched to one-way pricing within Europe is the emergence of low-cost carriers who sell one-ways only. Like in any other industry airlines try to maximize profits and offer cheap one-ways only if they have to in order to remain competitive. A good example is Lufthansa, who sell cheap one-ways only in those markets where competition is forcing them to do so (Air Berlin and German Railways within Germany and BA for Germany-London).