Since you are asking about ethics, not about law, I wont say anything about legal stuff, but I suppose you are aware that most countries have laws/regulations stating when you are allowed to (a) collect personal data and (b) send mass-emails to people.
Assuming that you will not pass those emails on to third parties and that you use them to send out information related to the community and that each email has an unsubscribe link at that people can click to stop receiving these emails, I would say it’s acceptable to do that. You could add a clause to your Terms of Service informing people about what you are doing, that way you can refer people to those terms if you get any complaints.
If you want to make signing up for the newsletter more of an opt-in thing, then yes, you could use a banner or a pinned topic to encourage people to do so. But I agree that it would be confusing to join a forum and then enter my email again to signup for a newsletter (because I would thing: “but you already have my email”). So a better option would be to create a custom user field “Yes, I want to subscribe to the xyz weekly newsletter” which will be shown on the sign-up screen. The only disadvantage with that would be that user fields are currently not displayed for people who sign up via an invite link.
A mid-way solution between the first option (opt-out) and the second (opt-in) would be to merely state on the sign-up page as well as the invitee welcome page, that “By joining the forum, I agree to receive a monthly newsletter”. That can be added via CSS.
Having said all that, I wonder whether you actually need a separate newsletter at all. It is a common practice to create category “Updates and announcement” (or the like) and have all new users watching first post of that category by default. That way you just post your newsletter in that category and you’re done.
The following topic might also be interesting for you: