There is, just many email marketing people leave it out to cut down on the unsubscribe rate. Subscribing has to be easy and in-the-face (like those 'helpful' popups on websites), unsubscribing has to be almost impossible. Not totally impossible, mind you, because otherwise courts get angry.

Better email clients know to use the header, and e.g. gmail even asks about properly unsubscribing when marking a newsletter with that header as spam iirc.