Since it's legal for me to take legal tender for software, for jpegs, for mp3's, etc., how could a law be passed to make it illegal for me to take legal tender in return for a bitcoin?
If I advertise that I want to buy bitcoins, how can they stop me from spending my dollars/euros on them? If I advertise that I have bitcoins to sell, how can they stop me from doing so?
Now, if I advertise that I both buy AND sell them, at some bid/ask spread, than perhaps I have blundered in to an area where they can, indeed, regulate me.