You're hiding highly relevant text behind that "[...]".
"...for any term of years or for life, except that the sentence of death shall not be imposed unless the jury or, if there is no jury, the court, further finds that the offense resulted in the identification by a foreign power (as defined in section 101(a) of the Foreign Intelligence Surveillance Act of 1978) of an individual acting as an agent of the United States and consequently in the death of that individual, or directly concerned nuclear weaponry, military spacecraft or satellites, early warning systems, or other means of defense or retaliation against large-scale attack; war plans; communications intelligence or cryptographic information; or any other major weapons system or major element of defense strategy."
tl;dr - the death penalty is only applicable if you expose our spies and they are killed, or if you communicate certain kinds of information. (Those listed are usually classified "Top Secret", which to my knowledge, nothing published by Wikileaks has been.)
You are right, no "top secret" documents were published.
Still there were some claims that some names were exposed in the documents. I can imagine that somebody from all the exposed names got killed in one way or another. Would that be enough then? If something like that existed, Manning is then the first to be sentenced to death.
People have tried, vigorous tried, to find a single case where a exposed name got killed in relation to the documents. So far, not a single researcher has found one. What they have found is that there is very few names like that, the most of the named people was dead before the documents was published.
I may be operating under a misconception, but can't you only be charged with treason if you are a citizen of that country? Otherwise surely he should be a prisoner of war, or if not covered by that convention an unarmed combatant.
This is espionage, not treason. Both are capital offenses, though it's not clear that it's actually legal to execute someone for these crimes in the US due to Kennedy v. Louisiana. (That ruling specifically refuses to comment on crimes against the state, but I'm sure the next person that gets a death sentence for one of these crimes will be quick to ask the court for a review.)
"...for any term of years or for life, except that the sentence of death shall not be imposed unless the jury or, if there is no jury, the court, further finds that the offense resulted in the identification by a foreign power (as defined in section 101(a) of the Foreign Intelligence Surveillance Act of 1978) of an individual acting as an agent of the United States and consequently in the death of that individual, or directly concerned nuclear weaponry, military spacecraft or satellites, early warning systems, or other means of defense or retaliation against large-scale attack; war plans; communications intelligence or cryptographic information; or any other major weapons system or major element of defense strategy."
tl;dr - the death penalty is only applicable if you expose our spies and they are killed, or if you communicate certain kinds of information. (Those listed are usually classified "Top Secret", which to my knowledge, nothing published by Wikileaks has been.)