It's just that myBB comes with the feature of allowing guests to post. However the string is kept the same always, it's not really useful to do something about it. If they did, they could simply strip that checkbox when the user's not registered, and it doesn't.
Well yeah, but as guests can't post anymore, it's useless to have the "registered users only" notice there, guests can't even see the newreply.php or newthread.php.