Snowden worked for the National Security Agency, an agency dedicated to the protection of U.S. interests and its citizens, and which requires of oaths of secrecy. Breaking those oaths alone, brazenly betraying the trust of one of the most important institutions for national security, fulfills the requirement for treason. Again, one might argue that his mactions be ethically justified, but those actions are treason according to any reasonable definition of the word.
In addition, Snowden’s leaks have revealed substantial details about U.S. capabilities and actions taken in maintaining U.S. national security interests, both domestic and abroad, which could give an advantage to potential adversaries. Treason.