Some interesting things coming up here. Regarding the anticipated Avantix software update, I can understand the reasoning behind removing the ability to swipe and sign or manually key in the card details, but it does rather leave the rail industry in an unfortunate position of having absolutely no means of extracting payment from holders of certain, often perfectly legitimate, credit and debit cards.
Regarding 'ancient' Dione kit, I was amused to find recently that Rail Gourmet staff on a certain route now have live chip and pin devices, meaning the trolley bod has better equipment than the Guard!
As for the original question, TOCs have good reason to take issue with very high value items being sold through Avantix on board. The systems in place at a booking office are considerably more secure and far less likely to fall victim to fraud. There is also the issue of a Guard being on commission and thus, possibly in some cases, rather more keen to issue the ticket than a staff member who has nothing to gain and is more likely to refuse the sale if there is reason to be suspicious.