Section 798 of the United States Code makes it a federal crime if a person “knowingly and willfully communicates, furnishes, transmits, or otherwise makes available to an unauthorized person, or publishes, or uses in any manner prejudicial to the safety or interest of the United States” any classified information concerning communication intelligence.
Fortunatly for Snowden, alerting US Citizens that their Government is acting in an illegal manner, against the Citizens best interest, is not a crime under this section 798
That went out the window when he went to China. If he told the ChiComs what brand of cigarettes his boss smokes he should be hung.