What is wrong with what Wikileaks did/is doing?

So far as I understand it, Wikileaks has published hundreds of thousands of classified documents, but the maintenance of confidentiality is the burden of the department(s) in question. Shouldn’t the efforts of the US government be focussed at plugging the leaks in their own departments rather than gagging Wikileaks?

I have always operated by the principle that if you have nothing to hide, the spread of truthful information cannot harm you. Is this just another case of revenge, because the playground bully got a little bruised? Democracy is supposed to be based on the wishes of the people – so why is the US government so scared of the people finding out how they have governed?