Doesn't the google spam thing affect the global standing of the sender? If that's the case it's probably the good internet citizen thing to unsubscribe without marking as spam if it isn't spam, otherwise it's negatively affecting the deliverability of someone legitimate.
This is a feature. If an email newsletter doesn't have a functioning opt out (one that works with email address forwarding , I must add), the sender is not a good internet citizen.
Please don't do this unless there isn't a link within the newsletter. I seem to get a couple of people that do this every month with my Hacker Newsletter. I don't think a couple hurts, but it can if it goes above some percentage of your subscriber size.
For everyone replying/panicing about this approach:
Google gives you two options when you hit spam,
1) just unsubscribe
2) unsubscribe and report spam.
I'm surprised those with newsletters haven't tested this.
Oh and if you auto-subscribed me for anything other than me specifically adding my email to a "subscribe" form - you deserve to be spammed. Signing up for a site isn't a newsletter opt-in by itself.
you should not mark newsletters as spam as a quick way of unsubscribing from them, since it will affect the score of the sender; other people may find their content relevant, since they subscribed to them.