They may have it. It's likely not a very difficult thing to add, but in 3 years, you're (I believe) only the 2nd user to request that feature. It is on our investigation list, but we simply haven't gotten into the details yet.
Finally, the CB method, from what I understand, doesn't actually delete your FB application from the user's profile, it simply dis-associates their Facebook user ID with the Joomla user ID in your site's database. While that does partially disconnect the user's FB account from your site, it really doesn't do much else. If we were to do that, the automatic login functionality would cause issues because we look to see if the user still has your app installed, and if so, attempts to log them in.
Hope that helps explain our current implementation and the proper disconnection procedure that we'd need to implement when we actually get to that feature.
Thanks,
Alex