[Thamnophis]

There are many people who feed their fiseating snakes only goldfish without problems.
In theory you will have to add Vitamin B1 (Thiamine) when you feed them.
But I know from people who never added any vitamins and gave their Thamnophis only goldfish. Has never given any problem.

I do not know what wisdom is in this case.

I personally do not feed them. But when I would feed them, I would give them as a part of a bigger assortment of foodanimals. Never would this be the only food they would get.