Originally Posted by cepheid

It most certainly can be, if there is no pre-existing debt. "Legal tender" must be accepted for pre-existing debt, for example paying for a meal after having eaten it. When debt is incurred cotemporaneously with payment, such as when pre-paying for food before eating it, the merchant is not required to accept legal tender at all, or may restrict the forms of legal tender accepted. See Wikipedia.

By that token, banks presumably do not have to accept all forms of legal tender, since there is no debt incurred. However, a bank isn't really a merchant, and they're regulated differently, so I don't really know whether or not they can turn away specific forms of currency.