Deceptive subscription of email addresses to a mailing list will lead to a larger percentage of emails reported as spam. This will in turn increase the chance that Gmail / etc. mailbox provider will take actions to prevent you from contacting users in the future. If the newsletter is an important part of your business, losing it could be devastating.

Sometimes a matter of ethics can be reframed in the context of long-term reputational risk. "X bad incident related to our reputation" can be a lot easier to attach a dollar value to than "being unethical", and that is (in the end) what many engineer-employing companies care most about.