Spam.
Mailchimp is a company that provides lots of unsolicited commercial emails. In other words, spam. You can dress it up and call it "marketing software for small businesses" but that doesn't change the essential fact: Mailchamp is a spammer. Is it any surprise that spamming is profitable?
I've received hundreds of Mailchimp emails. Not once did I sign up for any of those lists.
Does Mailchimp make it easy to unsubscribe? Sure. But that doesn't change the fact that they are spammers, and that if you want to send spam with some semi-plausible deniability that you're a spammer, Mailchimp is probably a good choice.
Of course, this story, like nearly all "business news" stories, is very likely the work of a highly paid public relations agency. That is one more reason that the word "spam" does not appear in this story.