It is a minor issue, when creating a new discussion for a 'container' category where no posts are allowed it correctly does not allow the post to be submitted, but there is no message at the start (for example when clicking on the category) to say "Posting to this category not allowed", or just not make container categories available for new discussion creation.
A member can waste time creating the post, not be able to submit and not know why, raising a potential support query.