It's a way to know that there's a real human behind the computer screen and that the address is not fake. Even a single bit of information can be valuable.

>Wouldn't that hurt their deliverability?

If you're referring to deliverability of emails in the context of email spam filters then no. Having an unsubscribe option on repeated, unanswered solicitations would be helpful. The emails are not spam, and in the first email chain they are rather straightforward about their proposal and methods. I'm not sure what the scam actually is here.. people offering money for dev to scam users out of bandwidth? I do think it's noble of dev ignore the solicitation and provide exposure to this market however.