I was asked today why is making public, sensitive info considered by so many a good thing?
It is considered a good thing because in a democracy the public is the source of all authority and therefore the public is the entity in charge. It is crucial that the entity in charge know what its servants are doing when they are committing abuses and mismanagement or illegal acts. Release to the public information that shows that the servants of the public have been lying, cheating or just f-ing up badly is a good thing..
Now to get picky, since that's what we lawyers do: Whistleblowing is the disclosure by a person, usually an employee in a government agency or private enterprise, to the public or to those in authority, of mismanagement, corruption, illegality, or some other wrongdoing. So if it is whistleblowing to release document A (depicting wrongdoing), that makes him a whistleblower, whether or not the contemporaneous release of document B would have qualified him as such. Whistleblower and traitor are not necessarily mutually exclusive descriptors.
Now as to the acquittal on the aiding the enemy charge: the key factor is releasing "to the enemy". Manning released the documents to someone at Wikileaks. That person is not the enemy. That person then said to the Pentagon "do you want to claim any of this is too dangerous to release or redact anything before we release it?" The Pentagon did not respond because they have a policy of not talking to Wikileaks at all. Wikileaks then made most of the documents public (on their own withholding about 15,000 they thought were too potentially dangerous to US citizens). Again, "to the public" is not the same as "to the enemy". Even if the enemy could see it, that does not qualify as indirectly [i]giving it[/i] "to the enemy". If I tell you a secret, and you tell your aunt Sally, and aunt Sally gabs about it to her hairdresser, and her hairdresser writes it down for later and leaves it on the counter and a member of an enemy organization walks in and sees it, I have not given secrets to the enemy. Even if I told you with the reasonable expectation that you are a big blabbermouth, it's not giving information to the enemy just because the enemy happens to hear it. Giving something to someone, either directly or indirectly, requires that you intended for that particular person (or group or subset) to receive it. There is no evidence that Manning gave material to Wikileaks with the intent that it get into the hands of alQaeda - particularly as he reasonably expected Wikileaks to try and keep the bad stuff from alQaeda, which they apparently did try to do.
p.s. I have not looked up the legal standard but it may not require specific intent, but just the reckless indifference to a known harm - which this still did not qualify as.