As was observed in the discussion of the Snap Cafe, you cannot force a retailer to accept cash. If, however, you buy something from them and there is no contractual barrier to the use of cash, and you offer legal tender in payment, and they refuse it, then they cannot enforce the debt in court. So if you incur a debt, you can discharge it with legal tender, but you cannot be forced to incur the debt in the first place, if you see what I mean. This came up again today: a Techdirt story about Apple refusing to accept cash for iPhones and insisting on credit cards conveniently had a link to the relevant U.S. Treasury page.
Incidentally, I heard on BBC radio that it now costs the U.S. mint approximately 1.7 cents to make a penny and an astonishing 10 cents to make a nickel. This isn’t (entirely) because it’s a government-run industry but because of the metal content of the coins. U.K. “copper” coins are actually steel with copper plating and so safe from melting down for the time being (but the face value of the pre-1992 copper coins is below the scrap value). No-one here will be melting down coins to make more valuable razor blades (as they do in Bangladesh) just yet!
These opinions are my own (I think) and presented solely in my capacity as an interested member of the general public [posted with ecto]