Wikileaks publishes source documents. All documents that aren't created by the US government are usually protected by copyright.
There might be some other countries which also releases it's document automatically in the public domain but if a company created a document that document is copyrighted.
If Wikileaks publish that document created by a company you can debate whether Wikileaks can do so under the fair use clause but in general the document is copyrighted. The source documents that Wikileaks publishes are no derivative works. They are copies unchanged comments of the orginal.
You can say that Wikileaks is engaging in fair use. I certainly think so. Other parties however might think differently.
Especially when it's their documents that are published and they are in a position to preventively block the IP address because they have economic control over an ISP.
While we are at the topic there a pretty good chance that this forum also hosts a few copyrighted documents that forum members who don't now US laws posted here. We moderators however don't have an ability to check in every case whether a post is authentic or copyrighted.
Some people uses copyrighted images as avatars. It however to burdensome to check in every case so we instead assume that posters know what they are doing and remove the avatar in question should we get a message by the copyright holder.
Nearly any online forum of a decent size hosts copyrighted content.
For some time \b on 4chan got censored in Australia because some of it's members hosted unlawful content even through they violated forum rules to do so.
Of course you produce copyrighted material. On this forum you wrote 87 articles on which you own a copyright (and this forum has a license to display the content that you transfered through signing up).
How about JoChem: Biosemantics Group - Erasmus MC / LUMC - The joint chemical dictionary (Jochem)
The American Chemical Society claims they own CAS numbers. Should some ISP be able to preventively shut down JoChem because JoChem freely distributes the CAS numbers?
That would actually block some important scientific research that uses the JoChem dictionary for textmining medical documents. I actually raise my hand on the issue of having used JoChem academically and having to ask myself whether or not I'm in that process violated copyright. I was interning for a sort of public/private entity so I'm not even sure whether the possible copyright violation would have been mine or a violation of the entitiy.