(Incidentally, if a tram passenger did alight at Wimbledon then touch
on one of the Oyster readers on the tram platform, then it's quite
possible there'd be no ill-effect - Tramlink allows one 'free
transfer' using Oyster from tram to tram, as any Tramlink destination
can be reached by using just two trams - this is implemented by
allowing a second 'free' touch-in at a tram stop within IIRC 70
minutes of the first touch-in. Therefore by my reading of things if a
passenger arriving at Wimbledon mistakenly touches on one of the tram
platform validators they wouldn't be charged for another tram journey
unless they've already used up their 'free transfer' - the same
applies for anyone getting off a tram anywhere else on the system and
erroneously thinking they need to touch-out.)

My understanding, based on my possibly incorrect recollection of old
discussions elsewhere and supported by another contributor to this thread,
is that actually a touch-in on arrival at Wimbledon would be treated as the
start of a new single journey on the tram and the 'free' touch-in wouldn't
apply. It seems to be presumed that doubling back via Wimbledon would not
be a legitimate single tram journey. I'd personally disagree as if I was at
e.g. Dundonald Road in unpleasant weather conditions I'd prefer to get on
the tram to Wimbledon and back rather than waiting for what might be the
exactly same tram going in the opposite direction. I might also like to do
it just to guarantee seat on the tram if I thought Wimbledon was likely to
be very busy. Presumably the same applies at Elmers End, Beckenham and New
Addington. I've quite often been able to get off at Ampere Way, do some
shopping and touch in again within the 70 minutes that gives me my return
journey for free. It seems because this loophole/benefit can be removed at
the termini, that is exactly what they have done. I think it would be fairer
if they left it in place to make it an even playing field for everyone
(except those at Wimbledon who have to go through an exit gateline anyway).