Actually it's not the "only" explanation, it's one of several. BAA would be anxious to prevent GIP and MAG giving long haul a toe hold at LGW and STN respectively. Of course this is not working out to GIP's plan at LGW and I think the risk is overstated at STN. No monopoly is ever willingly given up and BAA was no exception.
You may be right, Stansted my have helped pay for T2 and T5 at LHR, in the same way LHR paid for Norman Foster's terminal at STN. This was done by BAA at Group level and is common practice. Indeed MAN may well end up subsidising investment at STN via MAG. Not sure why you keep banging on at this point as if it wasn't common practice.