Forging currency is illegal even though all of the components and component activities are legal. (Paper, ink, printing, lithography.) The Feds (or any government) can shut down anything they like given the right legislation. Anything that allows end runs around currency restrictions and taxes is fair game for governments, and passing laws against it (or simpy extending existing laws) wouldn't even be unconstitutional in the US.

Did you declare all your bitcoin transactions to the IRS? You may already be in trouble...