The reason 99% of affiliate sites obtain a penalty is because there an affiliate site, yahoo dosn’t like affiliate sites fact and they have even admitted this. They tweaked there software a long time ago to pick up affiliate sites using too many affiliate links and apply a penalty to them.
There is a way around it and also a way to get back in, but i’m not about to disclose those details on a public forum, the way back in is nothing dodgy and involves creating a lot of unique content and other stuff.
The reason i’m not going to post details of how to get back in is because i’m enjoying less competition in yahoo.
The main issue here is to try and avoid the penalty in the first place, if a site has just a bunch of affiliate links on every page and cant offer the user anything else usefull then it deserves to be banned in yahoo’s eyes.