This isn't a remotely new or unique precedent. Mailchimp and every other email marketing company have always had lists of prohibited content categories.

https://mailchimp.com/legal/acceptable_use/

https://ui.constantcontact.com/CCProhibitedContent.jsp

https://help.convertkit.com/article/76-terms

If enough people report those mails as spam, they'll ban those too. They're simply protecting their business, nothing more, nothing less.

Every mail I receive that I didn't sign up for gets a spam report from me. With that in mind, every single ICO mail I've gotten has been spam.