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?
--
David of Broadway
New York, NY, USA