He who owns the computer determines what can or cannot be on it.
Except that it changes from base-to-base and most of the time it doesn't follow actual DoD guidelines. The issue is trying to get a base to un-block a site that shouldn't be blocked to begin with.
For example, at Kadena AoL news was blocked but plenty of other news sites weren't. Or any link dealing with wikileaks was blocked, but yet by DoD policy we simply weren't to go to the actual wikileaks website (note that it is legal to read news stories about wikileaks; the issue is in the wikileaks website having still-classified info) . But yet at my current base I don't have either of those problems.
Pesonally I always like the "Catagory - None" blocks. Always makes me go wtf.