They're not "censoring people for discussing a perfectly legal technology"; read their policy carefully:

>>> It’s important to note that this update to our policy does not prevent the discussion of related topics in messages sent through our platform. For example, journalists and publications may send cryptocurrency-related information as long as they’re not involved in the production, sale, exchange, storage, or marketing of cryptocurrencies. <<<

Unfortunately, the reality is: there's a LOT of fraud associated with the production, promotion and sale of ICOs, and it's no doubt creating expense, risk, and overhead for Mailchimp.

As a private company, it's entirely proper and reasonable for them to prohibit activities that directly affect their business.