Private companies aren't required to take dollars. They just have to tell you before they provide service. I.e. you can't provide a service and then when they try to settle up you say, "Sorry I only take payment in potatoes." You're totally allowed to say, "Sure, I'd be happy to serve you a steak dinner. However, I only accept payment in dryer lint." Because you can always just say, "Damn, I spent all my dryer lint at FedEx Kinkos. Thanks anyway!"

Legal tender laws mean local/state/federal governments can't refuse to take your dollars for fees/taxes.