Yeah, don't understand some of the responses which seem to assume this change will make MailChimp any more spammer-friendly. If you want to spam with MailChimp (or for that matter most other list providers) you import the long list of people you want to spam in csv form and disregard warnings about only importing emails from people if they've previously expressed interest in receiving notifications from you. Double opt in for new signups makes no difference.
MailChimp's ability to be regarded as a non-spammy mailing list provider depends mainly on them weeding out customers who import lists of people who haven't expressed any interest to indiscriminately blast mailshots, not on requiring an additional step after an individual actually visits a website, types in an email and clicks a button sending a post request to the mailing list.
The concern here is revenge-spam: someone takes your email and submits it to every mailchimp default form they can find automatically, and then your inbox is flooded with ostensibly legitimate email that you have to manually unsubscribe from, for each individual list.
I understand the concern on an individual level as expressed in the article, but doubt "revenge spam" even moves the needle with mail providers' decisions on MailChimp mails get through or not, which is the primary concern of people worrying about its deliverability.
MailChimp's ability to be regarded as a non-spammy mailing list provider depends mainly on them weeding out customers who import lists of people who haven't expressed any interest to indiscriminately blast mailshots, not on requiring an additional step after an individual actually visits a website, types in an email and clicks a button sending a post request to the mailing list.