But a journalist, should have the right to ask people to come forward if they have information with a public interest, even if it is classified. If someone comes forward, they may break the law, but not the journalist.
Further more, Assange is not a US citizen, and is there for under no obligation to be loyal to or protect US interests or secrets. He cant by definition be treasonous again any country besides his own.
If he on the other hand, provides material support to someone committing a crime, (like helping to crack a password), then he can be tried as an accomplice. This evidence seems to imply he did not.