Ah, but then fewer people would "sign up for the newsletter" by failing to toggle the box on the checkout page. Then your list membership growth rate would go down! Can't risk that, no wonder it hasn't been implemented.
Mailchimp turned off double opt-in as the default list behaviour, presumably as a result of this listbombing taking place.
It eliminates the immediate flood from the listbomb attack but does mean the email address now has to opt-out from every list they've been susbcribed to.
https://wordtothewise.com/2016/08/subscription-bombing-esps-...