I’ve looked into this a bit- I believe it’s related to the checkout page loading with a default of “Agrees to Marketing”.

What happens- at scale and I have to believe deliberately- is the “checkout created” event with that flag set to true is considered as “opted-in” by the marketing automation platforms everyone uses, like Klayvio.

Even if you immediately un-check it, un-checking doesn’t trigger an unsubscribe event, since you never submitted the form in the first place.

And because your Shopify session is now shared across stores, your email address gets opted-into marketing just visiting a checkout page.