I don’t think Snowden is completely immune to a charge of treason, but there’s certainly a distinction between the kind of thing he’s done vs. a Bradley Manning or The Rosenbergs (or, for that matter, John Kerry, the Clintons and Obama).
You could hang the vague “aid and comfort” charge onto him, I suppose, but Snowden hasn’t taken up arms against the U.S., and he hasn’t revealed specific classified material. The secret he’s revealed is essentially that there are no secrets. And I believe that his defense would be pretty solid in stating that violations of the Constitution cannot be protected by “classified” status or an oath of secrecy imposed by the violators.