The problem is that the cost of Blockchain / ICO spam does not just fall on Mailchimp as a company, it falls on MailChimp's customers, who have to suffer through reduced email deliverability (for the same cost they are already paying).
Other than that, MailChimp is a private company, not the government, and can damn well serve who they want.
But why should they? Doing that brings very little upside for MailChimp and a lot of potential downside if Google decides to blacklist MailChimp's entire IP range.
And then they have to devote engineering / moderation resource to routing emails to the correct IP shard.
Mailchimp has offered dedicated IPs for years - the code and operational processes around that are already established. Gmail also rarely blocks at the IP level, especially for an established provider like Mailchimp.
Ultimately, this is first and foremost a policy decision around the extent of fraud in ICOs generally, and secondarily related to many ICOs' lack of adherence to Mailchimp's anti-spam standards.
Other than that, MailChimp is a private company, not the government, and can damn well serve who they want.