Just sink this in...

Any retailer or individual could refuse to use a legal tender. Nobody can force you to use it. Why? What is the point of a legal tender then?

The point is that you cannot force a retailer to accept legal tender or indeed any other form of tender. If, however, you buy something from them and there is no contractual barrier to the use of any form of tender, and you offer legal tender in payment, and they refuse it, then they cannot enforce the debt in court.

That's what legal tender means: it's about discharging debts. 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...