What's fraudulent about this?
If you didn’t actually intend to travel, then claiming DR is fraud.
Easiest example is if you have a season ticket, but you have the day off. You weren’t going to take the train to work that day, so no intent to travel. If you claim DR, then that’s fraud for the compensation.
It would be better if the law was changed so that any transport company selling a ticket is forced to refund if they couldn't fulfil their obligation, regardless of whether the ticket was used or intended to be used. Can't provide the service? Then don't sell it!
There’s differences in consumer rights effectively between a refund and compensation (like DR)
It doesn't have to meet a legal definition of fraud for them to be able to shake you down either. They do plenty of fishing exercises and allegations without much of a strong case, hoping you'll pay up.
The government is very very hands off when it comes to the railway - law unto their own (except for the operators owned by the government - no escaping blame there!).
A lot of railway legislation offences are strict liability offences - ie there is no excuse and no proof of intent required. It is from there comes a lot of their bullying behaviour and a belief of what they say goes.