Bitcoin is not fiat, and the US current economic policies require that they be able to manipulate the money supply.
It is possible to create a cryptocurrency and still control the supply, but this would not be bitcoin.
Anything like that would ultimately be proprietary and under Gov control. Bitcoin is unique as it not only is designed to operate with no central authority, but it is completely open to public view as well. Anyone has the power to start their own currency with these open source tools, circumventing any "OneCoin" the government could create. Anything the government would do to stop it would be met with serious opposition, and much like the way Gov controls never stopped Bittorrent at all after spending millions, Bitcoin can't be stopped either without the Gov becoming North Korea in containing the Internet into a national Intranet, risking sparking a massive uprising. Last time someone even pointed a major, vague, and draconian legislation at the Internet, the response from Google and the tech industry was pretty much "do it an we'll shut this shit down ourselves". I think the Gov is assuming Bitcoin is a novelty and not paying much attention to it, for now. Remember, their Tech Czar or whatever in past has used phrases like "the internet is a series of tubes"...they have less than a clue about what this is or what it could become before it is too late.
I just hope it happens fast enough to where the Billionairs don't realize it and get stuck with Billions of dollars of effectively worthless currency, while the rest of us riding our BTC horse to freedom laugh all the way to the banks that no longer exist, because WE said so.