Email unsubscribe flows are typically binary: either super simple to unsubscribe (how it should be) or super convoluted (having to uncheck preferences, confirm email addresses, etc). This is a clever flow by Jetsetter because it is in fact super simple (just one click)... but it gently 'upsells' users: "How about just one email a month?" First, that's such a gentler, kinder way to ask for email access than having me check a slew of boxes.
Second, it comes after they have unsubscribed me - so there is established trust. When they say "just one email a month", I believe it.
Lastly, Jetsetter promotes their Facebook and Twitter pages. If you are going to lose an access point to your customers - might as well push other avenues to communicate and interact with them.