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 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 enforced to incur the debt in the first place.
Now is clear why they use this stupid term "legal tender" ?
What is the point of "legal tender" ?