I may be wrong about this, but I believe these spies are generally not formally charged with treason (probably for the reason I stated). Julius and Ethel Rosenberg, for example, were convicted of espionage and conspiracy to commit espionage, not treason. And Jonathan Pollard is rotting away in a Federal prison right now following his convictions on espionage charges, not treason.
Good point. Of course, leftist groups such as the VVAW openly publicized classified U.S. ship and aircraft movements during the Vietnam War without being charged with espionage.