The U.S are not going to make BTC a part of their reserves, there is no reason why they should do that, the U.S dollar is stable, and the U.S economy is a healthy one, so they have no business with BTC as a part of their reserve. BTC is decentralized, so direct control is not possible, they can only control through regulations, which they are already doing.
What you say makes no sense, because you can apply the same argument to gold, which is decentralised, and the USA does have gold reserves. In addition to regulations, one way to control it is also to get hold of a good amount.