Imagine going into your favorite bar or coffee shop or anyplace one might pay cash for a purchase and having a scale determine the value of your money. And how will you know the change you might get back is the correct weight? Just how big must your pockets be to carry "money." And what about the tip? Have to weigh it first? What happens if you go to another state? Just how will one carry around a bank deposit or withdrawal.
I rarely use my credit card, but a law like this would make the decision to use it easy.
Stupid is as stupid does.