Complain HN: Spammers and Content Length
This is an issue because the "Unsubscribe" link is always in an additional step (e.g., you have to open another 'link' to view the full e-mail).
This is exasperated when you find that Mariott's unsubscribe link is not 'aware' of the 'subscriber' email forcing the user to input the e-mail.
With GMail's Unified Inbox, you may not have be aware which e-mail was subscribed after all and will have to re-look at the e-mail and go through the steps again.
This is definitely a dark spam method.