Pop Quiz Hotshot!
You are SBC/Yahoo and you sell your users' email addresses and vital statistics to Spammers. Would it be in your best interest to block all Spam?
Of course it wouldn't, as SBC/Yahoo also likes to Spam their own users with advertising. So they make their Spam Filters very weak, and move any Spam it finds to the Bulk Folder instead of deleting it. I'll bet their NAV 2003 hasn't even been updated to catch the latest worms, so they also end up going to your inbox or bulk mail folder.