Quite. What utter nonsense.Against what law exactly. How would it be illegal to sell a ticket
It's probably 'just' that GWR have overlooked the need to instruct their TVM supplier to add the new Project Oval paper ticket type codes (DOR/DOS) to the whitelist. But that's a very substantial omission, certainly given how long it's now been going on, and one that they should apologise and reimburse people for.
Sadly, back in the real world, they'll fix it at some point when they feel like it and anyone who's been overcharged will have to go to extraordinary lengths to get a comparatively small sum back. So obviously most people won't bother, if they even know they've been overcharged.