I wasn't far wrong:
The peculiar thing is that the list of WikiLeaks followers is already public information. And just how useful is this list, anyway? WikiLeaks' Twitter stream is not protected, so anybody can ostensibly "follow" its contents without explicitly following it on Twitter, or even needing a Twitter account.
Essentially, anyone can see what is being tweeted by WikiLeaks, simply by browsing to http://twitter.com/wikileaks. Twitter won't be able to identify people who access this page through an anonymising proxy, so of course, the DoJ won't even know about those followers.