The check you give your grocer is denominated in U.S. dollars.
Amazing that this even has to be pointed out. The same being true with gift cards. Gift cards are accepted in payment only because someone has already spent the money to "buy" the card in the first place. When the gift card is presented as payment, the actual cash transaction has previously taken place, using US currency. The gift card, in such instances, acts more like an IOU than currency.
Bogus arguments always amaze me.