By that logic the government and the military could hide behind a liberal use of the "classified" label to hide any and all breaches of laws, regulations, misconduct, failure etc. It would literally break the proper functioning of the judiciary and the legislature, giving the executive sweeping powers to literally get away with anything.
Manning is a whistleblower. While she mishandled the affair and perhaps her motivations weren't purely ethical, that doesn't in any way take away from the fact, that the revelations caused absolutely no demonstrable loss of life, and revealed severe misconduct on behalf of the government and the military.