It seems to me that the Americans have nothing to do with bitcoin, but they may prevent the conversion of bitcoin into other currencies. This can significantly reduce the demand for it if it is not established free circulation of bitcoins.
Software is protected by the 1st amendment (freedom of speech), so blocking/banning Bitcoin would likely be considered an act of censorship meaning an attempt to ban it would be stopped by the constitution. I dont think that the US will be hindering Bitcoin anytime soon; in fact I heard it has really boosted in popularity on the west coast in the last few years.
Of course I can very likely see most of Europe, China, and Russia restricting or outright banning Bitcoin in a struggle to remain in complete control of their citizens.