In the US treason is very narrowly defined by the Constitution to be (paraphrasing) intentionally waging war against the US or aiding the enemy, confirmed by two witnesses to the act. It is quite possible that Snowden could be convicted of espionage but not treason, because we are not waging war with anyone, not even ISIS, therefore we dont have any enemies in this sense. The term treason when used in US politics is nearly always hyperbole.