Good senders (send mail people want, don't send unwanted mail, set clear expectations in regard to all aspects of email sending) get few "report spam" complaints.
Bad senders (who don't have permission, send too much, don't care about what their subscribers want, hide mailing expectations in privacy policies) get many "report spam" complaints.
There's a reason why ISPs measure based on this data. Because it works. Draw a line between "high complaints" and "low complaints" and you're accurately drawing a line between "wanted mail" and "unwanted mail."
Is it any surprise that ISPs leverage this data to paint an accurate picture of good mail versus bad mail, and decide mail delivery or rejection policies accordingly?
Q Interactive calls this a bug. I call it a feature.