If the US wanted bitcoin gone, they could have done so.
There exists something called an "51% attack", which means that anybody having more than half the hashrate of the Network is acknowledged as the boss and can do as he wants (stuff like spending the same bitcoin multiple times, manipulate the difficulty of future block mining, etc).
Back in April, before mass ASICS, the US could have trivially gotten that far. Hell, last year anybody willing to spend 5-10 million could have done it with off-the-counter hardware.
For an agencies who have a double digit BILLION black budgets, this is totally peanuts.