David of Broadway wrote:
MIG wrote:

A penalty fare is not a fine, and it should not be issued in a case
where fare evasion is suspected. It seems that the staff concerned
acted correctly and did not issue one in this case, because there was
suspicion of fare evasion.

It would have been wrong to issue penalty fare if fare evasion was
suspected, because it would make systematic evasion worthwhile (free
travel and the occasional £20 when caught).

Pardon my ignorance, but when is a penalty fare appropriate if not for
suspected fare evasion?

Only time I ever paid one was when I forgot my pass had expired - I
started from the DLR so no gate - of course I could not get out. I went
to the desk as soon as I realised my card was out-of-date. Given I tried
the card and only then did I know it had expired then it was hardly
evasion. I even forgot to argue that as I had only travelled on the DLR
I should have had a £5 penalty not a £10 one (it was some time ago).