"The statute...require[s] the government to prove that the defendant...communicated or made unavailable to an unauthorized person...in any manner prejudicial to the United States or for the benefit of any foreign government to the detriment of the United States...classified information..."

The middle part (labeled (3) by Popehat), as the blog correctly points out, is the reason this is a big controversy and not a straightforward spy case -- it's not at all clear that his disclosures were meant to benefit foreign powers, or that he was attempting to harm the United States.