It's because Yahoo wants to keep their advertisers. If your traffic is not producing any of their advertisers any sales then it is not converting and is then poor traffic for them. At the end of the day, if you advertise your site on another site and end up having a loss from it compared to what you spent then I am sure that you will not be happy about it as your advertising was an investment where you expect to gain on it and not loss.
I saw the email too....Its funny that I got the email, when I kept calling to tell them that the ads were not targeted to the content on my site....If the ads are not targeted...then how can they expect quality traffic....and I mean the ads were way off topic most of the time