It is illegal in the US to use foreign currency because it is illegal to use any money that is not denominated in US Dollars.
I am very interested in reviewing your evidence of this claim. Post here please.
You will be waiting forever. No one in the US has to use US currency at all.
Check out the links in the comment above and you'll find info that might be useful. You are right that nobody has to use US currency in the US (or any money for that matter), but "legal tender" means that if anyone refuses payment in US currency then the debt can be canceled by court order, which means they cannot sue for payment.