London Underground has nice clear policies around this, both about what you'll be charged

https://tfl.gov.uk/fares/how-to-pay-and-where-to-buy-tickets...

And about refunds (typically you'd get an automatic refund for a one-off event)

https://tfl.gov.uk/fares/refunds-and-replacements/touched-in...

I always have high levels of respect for companies with such clarity

In The Netherlands you get a full refund if you tap out at the same station within 20 minutes. If you travel with NS (National Railways), you even have 60 minutes to tap out.

Having someone pay just to wave off someone is incredibly customer-hostile. Besides, how many people are even committing fraud like that?

A parallel comment suggests it might be an “excursion fair.” Although it is not really the intended use of a train, some reasons an individual might ride the train might be sight-seeing, or because they are homeless and want somewhere warm to hang out. In that case, IMO it isn’t really a fraud attempt to get off on the same stop you got in, it is just unexpected use of the system.