Mark, you can fix 404s, but these pages have "200 OK" response in their headers indeed.
sorry for the possible confusion, I just called such pages "Not Found" because that's how they really should be called as I believe.
Adam (SBK) was first who mentioned this issue (and I have to admit I didn't catch it at once), but then he also linked to such mature blog post in the comment he left on my blog.
I don't know whether it's an intended behaviour or a true bug -- mature status and global tags are kind of a 'grey valley'.
not sure if I ever sent a request on this issue to the support, but taking such an opportunity a month ago I directly asked about this issue Andy who kindly replied in the thread mentioned above (and obviously had a chance to read Adam's note as well).
then I asked the same question directly Matt too, who was also kind enough to provide me with some details on the nature of Automattic's business.
and no, in both cases I didn't get any response from them -- it's probably either because I was too harsh or they were too busy to follow up those threads.
just in case if there are too many linkbacks to my blog they can be safely removed and relevant threads referenced can be found via 'google custom search' link on the sidebar (regular WP search doesn't look for comments and Pages), search terms are: mature empty.