The United States has considerable control over actions by its own citizens on its own territory, but not over noncitizens who reside overseas and work primarily in cyberspace.
But then, I suspect most of the bloviators know this. WikiLeaks is, for most of them, just a good opportunity to bash the Obama administration (as if George Bush would have been able to act any differently) without having to actually offer any concrete solutions. And what makes this especially great for Obama's critics is that there's not really a lot Obama can do about it, aside from bloviating a bit in return.
Bloviating aside, though, we should be focused not on Julian Assange, but on figuring out how to keep anyone from providing this kind of information to him in the first place. That's more boring, but much more effective.