OK I get your point.

But here's a solution that would work in the middle ground: still charge to
dissuade the fraudsters, but not as draconian as the £4 penalty.

How about, rather than being charged the £4 on top of your other daily
usage, it is cappable, ie the most you will be charged is (from 2007) £6.20
TOTAL which is the 1-6 cap rate.

That means that if someone goes from Plaistow to Mile End and forgets to
touch in or touch out they get charged £4, and any subsequent journeys
(including subsequent incomplete journeys that day) add to the amount until
a cap of £6.20

As it stands at the moment, the user will be charged £4 (or £8 if he/she
does it twice) and then on top of that could be charged £5.20 even if
he/she never goes further out than zone 3.

To make the cap £6.20 if there is one or more incomplete journeys would
keep most people happy.

Can you disagree with that?