I absolutely take that point.
But in our case, there was a TVM at Gowerton but it didn't have the Small Group Day fares loaded. So they went to all that expense, but they didn't do the job properly!
At Llanelli, the ticket office closed early. If the Guard hadn't been able to come round to sell us tickets (which in this case he did - just!), the TVMs at Llanelli wouldn't have been able to sell the tickets either, and don't have the option to select alternative origins.
On the Heart of Wales, you don't expect TVMs, and the friendly Guard (who I'd rate 10/10) had plenty of time to sell us the tickets, and talk about the excess problem too.
At Aberystwyth, I suppose one might reasonably expect the ticket office to be closed on a Sunday, however Avantix machines are incapable of applying the excess one of our group required, so he had to be 'let off', despite being willing to pay.
So, in some cases, the TVMs are provided but incapable of selling the correct ticket type, or origin/destination, or discount, or excess. In other cases, the Guard is equipped with an Avantix machine and both parties are happy for an excess to be paid, but the Avantix says no and so no payment can be made.
The rail industry appears to make it hard to pay at times, and that perhaps - wrongly - encourages some to not pay at all, and in some cases people who are willing to pay are denied the opportunity. It wouldn't happen in any other industry.