"Want to see if your users are opening their emails? Pay to display your message in gmail's promotions tab."

Now google has the data, and they almost defaulted the option for everyone. That gives them a big boost in data receives which they could use for marketing "(hey guys you remember you got × amount of image loads, we made your images get loaded by × more users now if you want the data we got it right here for the right price.)"

If you go to the trouble to figure out how to disable the "auto-load images" setting, then they're just as blocked as before.

If you accept the new default, it's far easier for them to track you, because Google will kindly be loading their tracking images on your behalf when you open the email, every time.