It is my understanding that there is a difference between legal and lawful. The government does not deal in the lawful realm but the legal realm. Legal implies contract and agreeing to terms. That's why paying tax is "optional". The government can only tax what they create and control (fiat, your securitized government issued identity). Since government did not create bitcoin, they can only tax it if you allow them to (when you file your taxes and declare your btc).