Don't take this for definate but I imagine it may be because if you are on a one way ticket then you are going to spend half as much on board as you would on a two-way flight. Therefore they can afford to drop the price of a return ticket because they know you will be spending on board on your return leg.
There will obviously be other reasons but i do know that a large emphasis is now placed on 'on-board sales' in order to make profit.
I'd be quite interested in the other reasons myself too seeing as i just bought a return ticket across the pond!
Regards
Foil