What Snowden did is, arguably, treason. Whatever you believe about the righteousness of his cause, or the evil of the programs he exposed, or the corruption of the government pursuing him, that he's being pursued is in fact a case of the system working as intended.
Not by anyone who understands the legal definition of treason. Among other things, treason presupposes formally declared hostilities. Even Cold War turncoats convicted of espionage for aiding the Soviet Union couldn't be charged with treason, since technically, we were never at war with the USSR.
He could be forgiven for not understanding what a formal declaration of war is, because the US hasn't issued such a declaration for the better part of a century.
That hasn't stopped us from starting or entering numerous wars, of course... they're just "undeclared." Cuts down on the paperwork, dontchaknow.
I do. I'm suggesting that will probably be the justification, regardless. What is treason and what gets treated as treason can be two different things.
Sorry, but even the most agressive prosecutor is highly unlikely to bring a charge that is so easily and cleanly dismissed. It's like trying to charge someone with the murder of a person who clearly isn't dead. Our justice system has gotten hyper-agressive, but it's not that bad. At least not yet.
A far more likely tack would be trying him under the Espionage Act, which is vague enough to make prosecution easy and defense hard. Relatively speaking, at least.