Who committed treason first, though?
Bradley Manning was the only person committing treason.
Actually, can you please explain why exposing someone in government who has done something illegal a treason? Why was exposing the people who outed Vallerie Plame treason? Why was exposing that Ronald Reagan was secretly selling weapons to terrorists in order to fund a private secret war treason? Why was exposing that IRS was targeting Tea Party members treason? Why did the entire Senate commit treason when they exposed Bill Clinton's private sexual relationship?
Or, do you even know what Bradley Manning actually did?
It was treason because he stole and released classified documents pertinent to national security. Period.
Yes, I would consider outing an active CIA agent (plume) as treason as well, and everyone involved should have been tried and imprisoned or executed.
The Iran-Contra scandal itself was an act of high treason, but the exposure of such was not (a downed plane uncovered the scandal and a subsequent investigation was launched.)
The IRS targeting Tea Party individuals is illegal and immoral, but not treason as it was not a threat to national security or a violation of the Espionage Act.
Clinton wasn't exposed by the senate, he was exposed by Linda Trip. He was also acquitted and his crime (perjury) had nothing to do with national security, so there was nothing even remotely treasonous about this case.