[dragonwriter]

> Isn't this technically illegal in the United States?

Nope.

> That is to say, all businesses are obligated to accept U.S. currency as payment

No, they aren't. If there is a preexisting debt, the “legal tender” status of currency has some effect especially if there isn't a contracted-for form of payment, but many businesses operate payment-first with no debt created, and those that don’t tend to contractually specify forms of payment.