That was an easy response. We recently had two visitors to London who wanted to do a single journey on the underground to get home from zone 1. Do they pay the ?4 single fare, or do they pay ?3 for a card and go through a registration process to get a card only for it to coast 50p more? And then have ?7.00 combined on their cards that they don't want on their cards?

Or how many tourists actually remember to apply in advance of thier visit for their 15 year old's Oyster photocard?

It's very easy to think that just because you understand Oyster that everybody else does. I was speaking to people in Hampshire recently who had never heard of it. When they come to London, they just go to the machine and buy a ticket - who tells them it's 150% more expensive?