That all seems pretty reasonable, and it's not like we have the right to demand that a private company host our content.
Think I found it in the ToS, though.
_"Mailchimp also does not allow the distribution of Content that is, in our sole discretion, materially false, inaccurate, or misleading in a way that could deceive or confuse others about important events, topics, or circumstances."_
Once upon a time, l'd consider this reasonable. They're within their right as a company. But anymore I wouldn't use their service if I was looking for a mail distribution platform.
I fully trust that the policy has been implemented with the best of intentions, but without a common core of cultural values, an environment of partisan hostility, and accepted narratives downstream from the raw data being interpreted as the absolute 'truth' by either side, I think it's safest to select contractors that you expect won't fold under political pressure, or engage in their own activism if your product is meant to be mass market.
Either that, or integrate a level of service redundancy to account for potential deplatforming from any of your providers.
If your product or service is meant to serve a partisan niche, I think you have a lot more latitude.
I don't care if adults look at porn or engage in porn-based commerce; I'm not saying "porn is immoral and it's good to block it!" I do believe that the vast majority of businesses not already working with that industry don't want the aggravation that comes along with dealing with that content, though, and that's why it's widely barred.
This is just one reason why people should not eagerly condemn everything that is "sexually explicit" and hand-wave it away ("Yuck, of course no one wants anything to do with that.")
On a site called Hacker News, people should grok that, but the audience around here isn't what it once was. Do you people want Gilead? Because this is how you get Gilead.
It's not insanely difficult to start your own mailing list/ newsletter service. At my previous job, I had did exactly that. Then you can absorb the potential legal liabilities yourself.