The "Posting Access" section should probably be more clear about the fact that it only affects entries, not comments. Maybe changing "post" to "post entries" in the existing text, and then add a paragraph along the lines of "This setting only affects who can post entries; it does not change who can post comments to those entries. If you would like your community set so that only members can post comments, you will need to change the comment settings for the community", with a link to FAQ 23 and a thing about the Work as User menu.
...because it's really not intuitive that "post" only means entries and not comments, and so community maintainers can get confused. :)