Things aren't so clear cut. Laws have to draw boundaries around things all the time, even if the particular location of any particular boundary cannot be easily

Things aren't so clear cut. Laws have to draw boundaries around things all the time, even if the particular location of any particular boundary cannot be easily justified, the necessity of the boundary's existence most certainly can. In this case, we can regulate electronic transactions, but we can't regulate cash. Furthermore, it happens to be the case that when you allow electronic transactions willy-nilly bad things happen, and it's harder for those bad things to happen when cash is involved (Breaking Bad would have been completely different if Walter White had hid is money in Bitcoin). So, we regulate electronic transactions more than we do cash. It doesn't solve all the problems, but it does mitigate them.