For a while now, Facebook has been showing deactivated accounts in one's friends list, which is nice. However, the total number of friends in my list, including disabled accounts, is one less than the "All Friends" count at the top. How can this be? Does Facebook count me as my own friend, or is there something else going on (that may or may not involve a National Security Letter)? I certainly want to know exactly who is able to view my posts, and so the implication that there's someone not on that list is unnerving. Privacy and security is the reason why deactivated accounts were made visible on the friends' list, so it would seem inconsistent with that action that there is some other account status that can allow someone to not show up in one's friends list while still being able to view their content. I've messaged Facebook support, but don't expect a response. Unless I confirm here this is normal, I think I need to take further steps, though I'm not sure what at this stage.
I've found that friends that disable their own account still appear (only to you) in your friends list and you can unfriend them; these friends can re-activate their account whenever they like. However if Facebook has disabled the account due to someone reporting it as a fake name or account, then they no longer appear in your friends list. If the person eventually clears this up with Facebook (e.g. by sending Facebook a copy of their ID) then Facebook may re-enable their account and they will re-appear on your friends list.
I agree with you that you should still be able to unfriend these people, but Facebook does not appear to provide a way to do that (other than deleting your account).
I got my answer after I filed a complaint with the Office of the Privacy Commissioner of Canada. Unsurprisingly, Facebook didn't ignore their query like it did mine. The answer is that, while accounts that users deactivate themselves do show up in your friends list, allowing you to unfriend them, accounts that Facebook staff disable (such as due to violation of ToC) do not show up. I presume, but don't know for sure, that such accounts can never be activated again, so it's not a security danger that one can't see them in one's friends list and unfriend them.