It is not a question of legality, really. If the Federal government and the executive branch in particular wants to do something, they just do it and make up the rules as they go. That's why the president can enter into war without congressional approval, bailout GM and cut out the bondholders, print money, and cheerfully violate every every amendment in the Bill of Rights whenever he wants or thinks he needs to. Same goes for Congress, the judicial branch and the states, only to a more limited extent.
They ALL pretty much do what they like and call it law, but it is really a dangerous lawlessness that threatens the end of the Republic. I know that sounds alarmist, and maybe it is, but you can't read the news every day and come to any other conclusion other than the Federal government seems to have no meaningful limits on its power.
What I'm saying is there is probably nothing wrong with Bitcoin under current law, nor should there be. However, if the government and the media decide they don't like Bitcoin, they can destroy it without even bothering to pass a new law by constantly investigating people, taking down websites, freezing bank accounts, and writing scare stories. You've seen this process happen in other contexts, like with Paypal or raves or rap music, or anything else they disapprove of.
The trick is for Bitcoin to somehow survive and come into wide use internationally before tactics like these can ruin it. If it gets big enough, Bitcoin will take on a life of its own and move beyond their reach. Then the nannies and statists of the world will just have to grit their teeth and live with it.