Peter Frimberley wrote:

It's only a tax on stupid tourists. Anyone visiting a different city,
especially those from overseas where English is not their first
language, is likely to bring with them a guide book or do some
internet research before they set off. I certainly always do, and that
research always includes checking out the public transport options and
fares in my destination city.

I reiterate my previous statement - you, like other readers of this NG,
are clued up on transport and travel. Not every tourist/visitor is -
and nor will they necessarily take a guidebook.

You also ignore the fact that typically newly arrived tourists come
with a huge pile of large denomination notes or travellers cheques.

Or credit/debit cards?

Quite how you expect them to fit those into Oyster machines is beyond
me.

I would expect a machine selling tickets costing gbp10 to accept gbp10
notes, and maybe even gbp20 ones. I don't know, but I wouldn't
consider it likely that many are turning up with wads of gbp50 notes -
though I may be wrong...

Neil