A part of the comment for the relevant code for this behavior:
We're skipping the "choose X" part and having it be implicit to your choice of resource
This departure from the rules as written was an intentional choice to simplify the user experience for this card. As you say, in 99.99% of games (I'd say more...), you want to get the most loyalty you can from the Casualty ability. In cases this edge-y, we consider the needs of the many to outweigh the needs of the few... or the one. #wotc_staff