The U.S. government moves very slowly. If Bitcoin succeeds, the time in which that will happen will be too short for the government to fully recognize the threat and issue an effective policy to oppose it before it (Bitcoin) becomes deeply entrenched in the economy and its benefits are widely perceived. At the point, the political opposition to such a move would be too great.
That is my impression. But it is also what I wish to be true, and I frequently try to remind myself to be careful not to be swayed by wishful thinking into self deception.