A trivial point, but I don't see that anyone's made it yet: one reason it's easier to report spam than to unsubscribe is that the UX for reporting spam is a lot better in Gmail.
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!