The vast majority of people don't care at all about read receipts.
Marketers don't care who _isn't_ reading their mail, they care how to send mail that more people will read.
Who subscribes to a direct email campaign (and doesn't unsubscribe, and doesn't flag as spam), and is still offended by the thought of the sender knowing it was read?
Those people can flip the setting back to prompt-to-show-images.
