I had a repeat of the problem at Moorgate of staff refusing to acknowledge that a York to London terminals ticket is valid through to Moorgate from Kings Cross via the underground. After the usual tedious argument the lady said she would "let me off, this time" but refused to acknowledge that the ticket was valid. If staff believe i was travelling without a valid ticket when why not prosecute me?
I suspect London Underground Customer Service Assistants (CSAs) can't report people for prosecution.
Up here in York you'd expect someone on a salary of around £34k per annum to be reasonably competent (and if they don't know something, to check, rather than make something up), and treat customers well, but the same isn't true at LU.
This is exactly the sort of behaviour I have learnt to expect from them (anything better is a bonus), and I can't see it changing any time soon.
Based on my observations, whenever people are "let off" on the railway, it generally means the customer was right.