If YAHOO KNOW that sites are harmful, why even include them in their serps?
 

"2 potentially harmful websites are marked on this page."

Unsolicited Emails
Harmful Trojans

Wouldn't it be in the best interests of surfers to just ban these sites right off the bat?

Stupid.

Because some may be looking for that specific site. They can't just ban all of them..

I would.

google does the same thing.

A lot of peoples sites get compromised via an exploit and the malicious code it injected. So there are sites which do not actually intend to harm their visitors. My assumption is that they leave it like that for a bit to give the webmaster an opportunity to fix the flaw without losing their rankings.

Because Yahoo shouldnt control the internet. Their job is to list sites, not to tell people what to visit and not.