> The acceptance of fiat in a transaction is not enforced.

I don't know what jurisdiction you're in, but if you happen to have any USD lying around, you'll see a note that says 'This note is legal tender for all debts, public and private'—this refers to legal tender laws which, in the US at least, means that if you refuse to accept USD for a transaction within the United States, then the debt is legally unenforceable. I don't see how that squares with what you've said above, but if I've misunderstood you (or there are different rules where you live), please correct me.

I think you're talking past one another. The thing you're taking about is specific to debts, but merchants are well within their rights to reject transactions, as no debt has yet been incurred. This is why it is perfectly legal to, for example, not accept $100 bills.

You'd think that, but not even the local DMV actually accepts cash dollars. They lack the security tools necessary to store and transfer them to the bank, so they've gone to requiring checks, money orders, or credit card

The use of fiat currency is usually enforced for debts. This doesn't apply to a cash purchase in a shop, but credit card purchases and most B2B transactions (with invoicing or a credit card) involve creating a debt.

As long as a means of payment is accepted by some creditor, it has value to the extent that it can find its way through the economy to that creditor. That’s why people (and money launderers) value Big Co gift cards at something approaching face value.

I am fairly certain that some jurisdictions do.