It would be pretty easy for the government to shut down any cryptocurrencies it cared too: go after the exchanges. This is already happening by making them follow the know your customer regulations that banks have had to follow for years. But I think you're being a little to cynical. I don't think the government cares that much about bitcoin except to the extent that it allows people to get around existing laws, and its high potential for abuse by unscrupulous business people.
