When you log out the main page reloads. I wonder if this makes sense? If I log out I am done. I don't need the browser to work on reloading the main page. How about a real small \"you are logged out\" page?
Logging in gives privileges to various portions of the site. Each type of account, such as registered user vs. member, has varying levels of access to tools. As listed above, in the case of the forums, a public user (not logged in) can read posts, but only registered users or members (logged in) can write posts.
So, logging out actually removes the privileges associated with the account that was logged in, thus the screen refresh. If the refresh did not occur, it would appear to you that you could still do things associated with your privileged account that would fail when logged out.
In the context of the site, logging out does not necessarily mean you're closing down the site, it just means you no longer have need of the special access provided by logging in.