Yahoo Inc. announced today that it will debut a service that will let direct marketers send targeted e-mails to consumers based on their search engine activity.
...Only people with Yahoo e-mail accounts who have opted in to receive third-party offers will receive marketing e-mails based on their search activity.
Emphasis mine. It's yucky technology (yes, that's a technical term), but You Get What You Pay For sometimes. A bit of browser config might be able to block this, if you're careful about what sites let you set cookies.