I believe Yahoo thinks that SPAM is effectively managed when placed in the user's Junk folder. That worked in the '80's with AOL. Now we KNOW that they are after much more than selling you a junk product/service. They are after viruses, profiles, personal data, behaviour, etc. etc. etc.
NOW we need to stop it at your servers - or give users ability to block the domain names they know are dangerous spam. Or Yahoo block the domains of spammers. Users will tell you who they are.
Don't give spammers the satisfaction of getting to the users box.
(Besides I use my junk folder for emails that I don't want to keep but want to see.)