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.
I can see this being a factor on budget airlines, but the likes of BA et al ? Surely most people dont spend much (and we are talking hundreds of pounds of difference in some cases) on board.
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chris