The Espionage Act is reserved for Americans who betray their country for the advancement of foreign adversaries, not for whistleblowers. In revealing the inner workings of the government's illegal surveillance of American citizens, Snowden humiliated the deep state and, as a result, was unfairly labeled a "traitor." This is an obvious case of whistleblower retaliation; it's no wonder he fled.
Karma can be brutal, and Snowden could very well be conscripted to fight in Ukraine or be under the Kremlin's control for intelligence operations. He certainly has "specialized skills" that would be useful to Moscow. Snowden sparked discussions about civil rights and privacy in the US, but his actions in exile - including his silence on Ukraine - are anchoring him staunchly as a US adversary.