The rumour I heard was that it was to thwart
@RJ using the 'through trains' rule for ticket validity, though I don't know if that's actually the reason.
To be fair it probably makes sense to advertise these trains to Sutton, rather than St Albans to St Albans via Sutton and the use of 'false destinations' en route.
Some of the other loop services may be more problematical in this respect, in that there may not be an obvious place to change the service ID.
As things stand, you can travel from Clapham Junction to Earlsfield via Kingston for the same price as a direct train. But SWR probably aren't bothered; a paper ticket for that journey is not cheap, and it's an irrelevance for PAYG.