When I was a contractor to the Air Force they simply said that the site had chat or bulletin board posting capabilities and was therefore a restricted site.
I'm currently an Air Force contractor and I recall it being blocked for a very brief period of time back around 1995 or so, then allowed to return without posting capabilities. Now, at least where I work, there are both reading and posting access. It's hard to see why they would block it now, given the wide-open access to YouTube and social networking sites.