Which is presumably done so that if you forward the newsletter to someone else they can't (accidentally or maliciously) unsubscribe you by clicking the link.
When it's trivial to re-subscribe if you want to, I'd prefer that they do include a one-click unsubscribe. I'll deal with anyone who maliciously unsubscribes me from a mailing list that I want to be subscribed to, and this will highlight who my "good" friends are anyway.