I don't get the argument that this is unfriendly to visitors. As a visitor to any other city, I find out how the fares work before I use public transport and I'd much rather use cashless systems than struggle with unfamiliar coins. However, I do see the need for one-day tourist cards rather than forcing people to pay £3 for a plastic Oyster.

Drivers already have the discretion to allow people on without paying - if my daughter ever forgets her Zip card (Oyster for schoolchildren) the driver gives her a ticket which states the fare she owes and how to pay it. I always pay it for her (even though the Zip card gives her free bus travel, because she should have had it with her, so it's fair to pay), and the man at the other end of the phone is always astounded that I do because no one else ever does. They have no way of pursuing anyone for these owed fares.