You're missing the point. Under US federal law, if I owe you a $100 debt and I hand you $100 in cash paper money then I have settled the debt. You cannot then sue me, demanding to be paid via Visa or PayPal or whatever. But this applies to debts only, not purchases.

I think you're missing my point! I'm not saying that the law doesn't actually require this; I'm just saying that there's nothing in general saying that something printed on every bill has to be true. In this case, it happens to be, but saying that it's printed on the bill isn't what proves it.

Nonsense. What's printed on the bill absolutely proves it. What kind of fantasy world are you living in?

Eh, "This note is legal tender for all debts, public and private" doesn't leave a lot of room for nuance.

My point is that the writing on the bill isn't what actually makes the law enforceable. If Congress passed a law that required the mints to print the words "this is a flamingo" on every dollar bill, it wouldn't make it true. The fact that cash is required to be accepted is because of the law passed, not because the magic words on the bill say so.

The writing on currency doesn't have to be. In this case it is. Or rather, the federal laws that back up that particular writing are.

In the US at least.