I don't know about his other leaks but there's evidence that he communicated with, arguably collaborated with and advised, Manning before Manning copied the classified material. If Manning just sent the material to Wikileaks, Assange would be in the clear (e.g. the Pentagon Papers, Trump's 1995 tax return info this week). Maybe it's not a solid enough case to be sure of a conviction, I doubt the U.S. would want to take a chance on it, but it's not like there's nothing there.
No one has suggested charging him with treason. He certainly can be guilty of espionage against the US without being a citizen.
(And his citizenship is irrelevant to whether he can exceed the protection of the 1st Amendment, since that applies whether or not he is a citizen with regard to government action such as prosecution for espionage.)