That's because of the unjustifiable rule that the tickets are considered to hold no value when used on the wrong TOC (revenue allocation is not a good excuse; Virgin have to excess "via High Wycombe" fares used on their services via Milton Keynes, for example!) and therefore the TOCs are putting their staff in a position of unnecessary conflict where they are told to charge not just a reasonable excess fare, but for a whole new ticket, which is not acceptable and will lead to customers being understandably unhappy!