When a site is penalized by Google, why isn’t a message sent to the owner via Google Webmaster Tools telling how and why it was penalized? Currently, site owners are left grasping in the dark trying to figure out what they did to cause a penalty.
The fact is we do. We don’t do it for all penalties because there are some bad guys, some spammers, and blackhats who you don’t want to clue in. But for things like hidden text, if we think your site is hacked or even vulnerable to hacking, we’ve even talked about sending messages to people who we think have old Google Analytics packages. But we do send a large number of people warnings about their site to sort of say “Hey, heads up, we’ve had to penalize your site because of hidden text or for other reasons”.
We don’t do it for every single site and we don’t do it for every single penalty because there are people who try to see how far they can get away with it and as soon they’ve been caught and they know that they’ve been caught, they’ll start again on another site. We’ll keep trying to find out ways to let the good guys know that they have issues without cluing in or alerting the bad guys, but we’ll never be able to tell or probably never be able to tell every single person that they’ve been penalized. We’ll keep trying to do a good job, though.
by Matt Cutts - Google's Head of Search Quality Team