i'm not sure for that but maybe about the permissions level.
When you log at administrator, go to Permission. You can see differents levels and differents options. So if i select some options with " Registered user with email confirmed " i think the simple registered user can't access to the options seleted.
1. Answering questions: Registered user with email confirmed
2. Adding comments: Registered user
Than, the Registered user can only add comment, but not autorized to Answer a questions
Maybe someone can confirme this