You see them because you've selected "showdead" on your profile page. They are dead because the person exhibited a lack of understanding of the mores of the community, and either posted spam, or egregiously violated common courtesy in comments.<p>Sometimes, I imagine, users may get auto killed prematurely; if their first submission appears to be obvious spam, for example. But, I wouldn't think that would happen often.
Check out this user's threads page with showdead on: <a href="http://news.ycombinator.com/threads?id=wernerv0nbraun" rel="nofollow">http://news.ycombinator.com/threads?id=wernerv0nbraun</a> -- all of their comments get deleted.<p>If the site has a honeypot system that shows the autodeleted comments only to the user that made them, that's clever, if a little much.<p>If it just instantly deletes them, that's lame, it'd be better just to ban him.<p>If this is being done by a moderator manually, that's fucking awful, and the moderator should be banned.