I think they're considering him "press" from the standpoint that he's publicly distributing information after a sort of review process (which makes it technically "publishing"). What material is being distributed is irrelevant in that definition, legal or not. Thin, I know, but when it comes to free speech, the protections have to be broad, otherwise bloggers and various other forms of writers and distributors can be nailed to the wall for their opinions.
Having said that, Assange doesn't live in the US, so our legal definitions don't apply anyway.