Mark Hynes wrote:

There's no difference surely between that situation and someone with a
paper
travelcard exiting outside their zones, surely?

Actually, there is a difference - with a paper ticket the passenger
will (likely)
know they're outside their zones and should be paying something extra,
somehow.
A passenger with both a travelcard and PAYG credit on their Oyster
card is likely
to think they'll have some credit deducted when they leave and not
have any idea
what an OEP is. (I know this has been done before, but it's still
crazy)

As you say, when challenged by an RPI the guy outside of his paper TC zones
has no defence - it's a fair cop. OTOH the 'Oyster passenger' you describe
can say 'I am going to touch out on exit' if challenged, but still not
bother...

Paul S