But I know from industry experience that some people who explicitly sign up to mailing lists then go on to report those emails as spam, and this dilutes the effectiveness of 'report as spam' features.
To this day we have never sent a marketing email, or even non-transactional email - mainly because we suck at marketing, so it's not like they got confused about which email...
^ We do audio transcription, a customer sends an mp3 in, we have someone listen and type it up, and we emailed the customer the results. These days you can collect the transcripts from the website, but for years you couldn't.
An unsubscribe button = "now stop emailing me." i.e. the opposite action of signing up and giving out an email.
flagging the kinds of things OP talks about after giving out your email address is ridiculous. Giving out your email address is opting in to communication initiated by the other side, without further requests - otherwise you wouldn't be giving out your email address, they would be giving you theirs: "Email us to get a reply with our newsletter" or whatever else the OP imagines in this bizarro-world where you opt in to a specific communication.
And that's why I wouldn't flag them, because I solicited the email.
But I wasn't talking about that case. And you know it. Please stop.