Personally, i've never understood why airlines do this. I can understand them charging more for a o/w than 50% of a return, but i've never figured out how on earth they can justify charging 1.5x the cost of a return for a single?? Surely they are actively encouraging people to do this. If they made a single 75% the price of a return, then they would get less people trying to bend the rules, and probably make more revenue on the single seats in the long run.