The List-Unsubscribe header was pioneered by Dave Rolsky, one of the more notorious spammers of the early 2000's. His reasoning was that most people were just going to hit delete, but anyone who went out of their way to unsubscribe was a squeaky wheel that would cause more problems for him if they got angry about their request being ignored. So he really did honor unsubscribe requests ... at least until adding them to the next spam campaign on a different list.