All of the posters on this topic make very valid aruguments. I like Scumbag O'Reilly's angle of viewpoint, but I don't think the drawing of a parallel with a restaurant menu would pass muster either at an airport or in court. As has already been pointed out, it is misguided to attempt to apply logic to fare rules - it just doesn't work!
You see, restaurants do not have any political pressure applied when setting their tariffs or any international agreements.
In any event, most airline reservation systems are such that failing to show for one sector results in the rest of the traeller's itinerary being automatically cancelled. Certainly in BA's case, no ticket agent would reinstate a booking without recalculating the fare and recharging the new amount - in the present climate, you'd probably end up on a disciplinary if you waived the fare rules!