WTF? There is an IETF standard for certifying that an spam victim's email address is valid and therefore should be exploited by sending more spam?

I don't have actual numbers, but a cursory examination of my Gmail spam box says spam senders don't generally provide a valid List-Unsubscribe header. One of Gmail's spam triggers may in fact be, someone clicked on the List-Unsubscribe header and the FBL message bounced.

Huh? How did you conclude that?

Tracking image loads and clicks would be a much more effective method of determining that an address is valid for both legitimate mass emails and spam.

Does Googlemail web client load images from random senders without asking the user first?

Only if you've previously exchanged messages with them.

How many people do you think load images versus click Report Spam on an ISP with a feedback loop? And which group do you think would be a better target for future messages?