The kindle puts my Calibre news items into the periodical folder once a new "issue" arrives so it definitely isn't treating it like a book. I've always deleted them manually so I'm not sure what would happen. You say 7 was the magic number? I guess you could just pick a daily news source and wait 7 days
I'll be watching this thread to see what is discovered because it's a pain deleting old periodicals manually.
Oh and FYI when you're reading one of the calibre news articles, if you hit menu there is an option to "keep issue" so it really does seem to treat it correctly. I just don't have any official amazon subscriptions so I'm not sure what it normally does.