The filters are so good, that many people find that they can't even reach people that have asked them to subscribe to their mails, because they are still filtered out as spam. Enter MailChimp, the trusted mass mailer provider that help companies bypass the spam filters by giving their mail a respectable envelope. If they allow spammers their envelope shouldn't be respected. Actually, that's clearly the theme of the OP.
And no, by using email I consented nothing. Where I live that's actually the law - I can sue whoever sends me commercial unsolicited email, unless given explicit permission (opt-in), spammer can be liable for compensation for each and every incident.
If it was open you would probably be receiving hundreds of them a minute. The filters means many don't even try, but if the required effort was zero we would be flooded with them.
The situation was so bad before the filtering products were invented that Bill Gates tried to push small fees (stamps) as a requirement for sending emails (https://www.cbsnews.com/news/fee-based-e-mail-way-to-can-spa...).
I feel your pain, but it's worth noting that email, by its very design, is intended to be open to receiving unsolicited messages. So frankly, by using email at all you have actually consented to receive unwanted emails from time to time.
I think it's unreasonable to expect MailChimp to maintain a ruthless approach to prevent people from doing something that can be done with any email client, sendmail, or any number of tools that use the open protocol of email.
Things are bad enough as it is with closed and segmented communications methods on the rise. I'm pretty happy that email exists the way that it does, and MailChimp strikes me as a company that really works hard to strike a balance between the competing needs of being useful for sending mass emails while being mindful of spam.