It is leaked by people onto his website (wikileaks.org), you can submit stuff now too. He simply times the release of said info and makes sense of them. A lot of the stuff sometimes is useless but at times he gets bulks of very important information. This one was leaked by - people are guessing - as the same guy who leaked the previous war logs. This batch of information was available to around 3-4 million us citizens and its not something very very "top secret". But it is highly embarrassing and does give an insight into foreign relations between countries.
Anyway he doesnt use any amendments. He is not American and not subject to their laws. He uses very secure hosting in countries that have proper freedom of information acts (Iceland etc).
The consequences of most of the leaks are simply embarrasment on America's part and wider public understanding of countries relations. The "They are terrorists" "People lives are at risk" is complete bullshit. They said the same thing before the Afghan and Iraq war logs, until now they have yet to find someone who was at risk because of the leak. Its the same with these diplomatic cables. Governments use those words to simplify concepts so it can become ready soundbites that are sprouted everywhere. I say fuck them, we all know goverments are walking hypocritcal institutions. At least now it shows them for what they are. "God Bless" the internet. At least now they cant suppress the information the public receives.
