FWIW the conditions of use for Oyster
http://www.tfl.gov.uk%2Fassets%2Fdow...uL_YIr_fB1vJpg
do make the distinction between what you "should" and "must" do:

"3.16 Oyster route validators (pink card reader)
Oyster route validators can be identified by a pink card reader:

These are located at a number of interchange stations. If you pass an
Oyster route validator when changing from one train to another, and you
are using your Oyster card to pay as you go for any part of your
journey, you should touch your card on the pink card reader to ensure
you pay the cheapest appropriate Oyster single fare for the route you
are taking.

You must still touch in on a yellow card reader at the start of your
journey and touch out at the end to ensure you pay the correct Oyster
single fare."


(I was sad enough to check when they were introduced at Highbury &
Islington.)

--
Robin
PM may be sent to rbw0{at}hotmail{dot}com