As per YSM Blog,
Yahoo! Search Marketing is rolling out a new feature that we think will help enhance the quality, potentially reduce the cost and increase the value of traffic to you, our advertisers. It’s called quality-based pricing, and it measures the quality of traffic coming from our distribution partners—that is, the web publishers large and small that display your ads.

More Here

At least this is fair for the advertisers and also the publishers .
If a lots of traffic go into your website , but most of them is not the quality leads ,that's no point for your website . All these traffic is just for stop - by . They maybe won't come back tomorrow .
The important thing for a business is repeat business .