Earlier quoted context omitted.
The answer is much simpler than you are making this appear. Sending an email is a private communication. Sending a tweet is a public communication. Your email provider is not a publishing platform any more than the paper store or the USPS is a publishing platform. Twitter is a publisher, although as an Internet publisher disseminating user generated content, they are protected by law from the typical liability that p…
I believe that's an artificial distinction. Companies like Mailchimp have already started implementing protections like mandatory opt-out mechanisms to protect themselves from liability. If networks of mailing lists around political rather than business interests were more common, I'd expect no less this kind of self-policing. The OP hits at a fundamental problem. I'm not sure there's a clear path forward.
Twitter, on the other hand, is publishing these messages on their site. It's very easy for them to simply stop publishing those messages. Because Twitter is in charge of Twitter, it also has to be concerned about it's brand. The more public nature of Twitter also help this along, of course.
If something like this was to happen in the ActivityPub or XMPP space, the way of handling it would probably be something in closer to email than twitter.