Cost-benefit
I can't imagine how many people actually click on one of these ads and buy something advertised through spam.
Mail advertising used to be a tightly run business. A certain percentage of responses was needed to make the effort worthwhile, and it was not an insignificant percentage. Not enough, and the ads stop.
E-mail has made advertising costs approach zero, so almost ANY response becomes cost-effective. What does it cost you to send a thousand e-mails? A few cents? Even ONE customer puts you in the black.