Ask HN: Are Square's marketing emails legal?

For a long time I've been annoyed by emails from Square (including unsolicited receipt emails) but I wonder now if they are even legal. Case-in-point, a marketing email I just received from a restaurant I visited once some time ago.

By what process did I opt-in to this? Is there fine print hidden somewhere in every Square kiosk that you adopt simply by allowing a merchant to swipe your card? Did I unknowingly opt-in to these marketing emails?

Square's website is of little help. In the spam email I received, I followed a link near the bottom next to the unsubscribe link that said "Manage Preferences." There is a link on the page with the text "learn more about marketing," which merely takes me to some other page called "Redeem your Square rewards." Is there even a copy of whatever I may or may not have agreed to posted on their website? I can't find any such thing in the "Legal" section of the site.