It is. Our constitution is pretty clear that what Mr Assange is doing counts as Freedom of Speech. The crime is commited by the people who leak the information to him.
Which is also true under US law. See the publishing of the Pentagon Papers or Bob Novak exposing Valerie Plame. Neither the NY Times or Novak broke any laws.
How does what Assange did not fall under the statute that I cited?
It probably does fall under the statute, but the statute is (or rather could be*) trumped by the whole "freedom of the press" business in the first amendment.
* If Assange is caught, this will go all the way to the Supreme Court. I'd be surprised if they didn't come down on the side of the first amendment.
