The internet is global. No such thing as "offshore" when dealing online. Look what they did to KimDotCom. Full US swat team landed on his property in new Zealand as if it were some slum village in Vietnam.
They took all of his property, froze all of his assets and told him to f** off. The only reason he isn't in jail right now is because he is still able to pay for a credible group of legal advisors
Google, apple, "big pharma" are all huge lobbyists and donate hundereds of millions toward 'their own' political causes in the united states. So you're right. America "isn't able to take control" of these guys, because it is these corporations that control America
If the nsa felt the urge to infiltrate bitcoin, they will have already done this even back when bitcoin was nothing but a seedling of an idea.
Look what they did with the silk road. They didn't wait till it was some rogue rival to GlaxoSmithKline, they penetrated very early and after robbing enough btc over a period of 'years', they brought the entire thing down and made themselves look like the heroes.
Also, the nsa might not want to destroy bitcoin at all.
Bitcoin is actually the wet dream of an agency like the nsa