Compounding this problem, conventional security wisdom is that you should never acknowledge unsolicited email, because the spammer might be using a fake unsubscribe link to confirm your email address is real. So a system that requires manual unsubscription this way will actually punish accidentally subscribed users for following good protocol.

Furthermore, if the "confirmation" email winds up in a spam filter and the user never sees it, subsequent emails will still go out and probably be auto-marked as spam.