The default permissions on the discussion tool are set so that "access" has the following permissions: new, read, and revise.own. This should also include new.topic
Without new.topic, students cannot begin a new conversation, all messages must be in reply to an existing message. This is contrary to how most discussion boards work. Most tools prevent students from creating new categories, but allow the creation of new topics (or " threads") within the category. Here students can create as many categories as they like; they just can't put any message in them.
By default, Sakai forces the maintainer to initiate each conversation. This flies in the face of standard practice, where the discussion tool is most frequently used as place for the students to ask questions outside of the classroom, begin group work, post essays for peer review, etc.