Biochemical signal transduction networks, consisting of receptors and enzymes, are very similar to artificial neural networks, in terms of computational structure.
Your argument makes sense when you view such transactions as a decision problem with a single actor and no temporal element.
But such transaction happens within a lifelong game with at least two players. Changing the rule of the game can influence their game tree and their behavior, far before they get into a situation where the change applies.
For the kidney trade, banning it might discourage potential organ-buyers from 'check-mating' people into selling their kidneys. It also might nudge parents to work harder since they have one less things to sell off.
I acutually have no opinion over banning organ trade. I'm just saying that it could make some sense.