Despite Bitcoin being called a currency, what gets lost about Bitcoin is that it’s a commodity–like gold or tulips), whereas a sovereign currency, such as the dollar, is what you use to pay your debt to the government (also known as taxes). The Treasury won’t accept bitcoins as payment (though they may confiscate property holdings, including bitcoins, and then auction them for currency). This doesn’t mean bitcoin can’t be used for trading.
At some Boston colleges, there is “Fenway Cash“, which allows you to buy currency that can only be used on campus or at stores in the Fenway area. One could, in theory, set up a complex economic system based on Fenway dollars. But when tax time rolls around, somehow these Fenway dollars must be converted to U.S. dollars. In other words, both Fenway dollars and Bitcoin are bartering mechanisms among individuals.
So it’s dollars or punishment, though I have no idea what would happen if tens of millions of Americans went on a tax strike (certainly, you couldn’t arrest or punish all of them).