I am not a lawyer, but it seems to me that leaking information and leaking the stolen documents are two different things. If it were just information and not stolen documents then the story would be different.
The US Supreme Court ruled in the Pentagon Papers case in 1971 that the government cannot order a media outlet to not print something on the grounds of secrecy as that would be a violation of the freedom of press.
I hate to say I agree with that ruling but I do. We are far better off with the government not being allowed to dictate to the press what it can and cannot publish. But even if it was legal for the government to censor a media outlet, even that would not have stopped WikiLeaks.