But it does not necessary mean that main user base is at US, for bitcoin and usually for trusted, popular altcoins, they are distributed the same in every place on the planet.
Most of the people will probably agree with me, with saying that all cryptocurrencies need decentralization, because we cannot let anyone to control them. If it will be able, then in my opinion there will be no difference between fiat and this kind of coin: after all, only market laws such as demand and supply should have control on it.