At no time has Assange had a US security clearance. He has no legal obligation to not publish info others have provided. Those others (Bradley Manning, Edward Snowden, etc) are legally liable for leaking information they were legally obligated to protect. Not Assange.
The US would have to prove that Assange directed the leakers to collect and transfer the leaked information to him. Other than this article I haven't seen any indication of such a level of control or oversight by Assange on his sources. They have chosen to commit espionage and have voluntarily chosen WikiLeaks. This article brings forth a claim of Bradly being directed by WikiLeaks but I have not seen that before not even during Bradley's trial and considering CNN's current reputation for creating "news" I doubt this unsubstantiated claim. So I'm having a hard time seeing how they charge him with anything that could stick.
As to the Clinton campaign emails, last I heard WikiLeaks still insists they were provided by a disgruntled DNC staffer, not the Russians. But even if from the Russians, WikiLeaks did nothing illegal. They simply published information they had been provided, regardless of the source. They did not steal the data.