The vast majority of "newsletters" I get today are from companies that assume that I want to keep hearing from them just because I once did a one-time transaction with them. And no, it's not because I forgot to uncheck the "I want to receive blah blah blah" checkbox. I'm careful to opt out whenever there's an option for that.
I don't know about other people, but I call that stuff spam. Just because it isn't an e-mail from someone who got my address from a dump and decided to cold-mail me with their crap (or phishing or other scams), doesn't mean it's not unsolicited e-mail, i.e. spam.
Maybe things have changed over the years and I just wasn't paying attention, but I've trained myself not to press "unsubscribe" on something I didn't explicitly opt in to receive.
Spam is for any kind of email I haven't opted in to receiving. That includes mail from organizations I have never requested contact with, as well as any emails I get as a result of UX dark patterns. I try to opt out, but some of them sneak past, and I don't feel bad dinging it as spam. That's the risk you take with opt-out mailing lists.
Another is if you get signed up for multiple different streams of messages ("What's New!", "Tips to Get the Most out of X", "Company Communications", "Coupons", etc.) just by signing up for an account (and, as you point out, not actually opting in for any of them explicitly). Since Gmail does not always flag all of these different things as spam, it can be more efficient to go to the "unsubscribe" page and uncheck each of these streams yourself.
If I want to politely unsubscribe from a newsletter I have to: switch from the keyboard to the mouse, hunt for the link in the email and click it, switch contexts to the browser tab it opens, hunt for the primary action on the page, figure out whether that action will confirm the unsubscribe, or whether clicking the button will sneakily re-subscribe me, and then close the tab.
On the other hand, if I report it as spam, I just press "!" one time and (in theory) I'll never see its ilk again.
Reporting something as spam is such a clean experience that I'm trained to use it for things it wasn't meant for, like unsubscribing from a legitimate newsletter. I suspect that this is true for other users as well!