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.

I definitely think thats a more realistic example of the things that might happen. It won't be the shady guy on the street corner, it'll be your boss asking you to do just one little thing.

While working in billing IT at a CLEC, I was asked to produce a phone bill for the CEO, with a printed physical address that didn't correspond to where he actually lived. I assumed (and still do) that it had something to do with a school district scam: he wanted his kids to attend a good school, without paying extra tuition. It was easy enough to do, and I don't feel there's much about the funding of USA public education that's worth defending anyway, but I can see how it could possibly have led to a series of increasingly immoral/illegal escalations.