Hi Mary,
Old post, but the bug still exists so here's some more details: all logged in users are in the "authenticated user" role. If this role is given ALLOW permission to "reply to posts", any logged in user should be able to reply to a post without needing to join the discussion.
In actuality, logged in users who aren't enrolled are prompted to enrol in the course in order to reply. I've upvoted the bug.
Regards,
David