It's even more than that. You are legally required to accept the national currency for the settlement of debts. If you are a US entity and we have a contract where I owe you 1 btc, you must accept the equivalent amount of dollars as settlement of the debt.
You are allowed to accept payments in different currencies, or even barter, but you can't require them.
You are allowed to accept payments in different currencies, or even barter, but you can't require them.