I know this is standard practice for most big companies moderating lots of content, but it has always seemed like such an insane policy to me.

Imagine if this were applied to actual laws enforced by the police. "You're under arrest but we won't tell you what law you've broken, because then other criminals might use that knowledge of the law to avoid being arrested. And by the way, a secret court has sentenced you to life imprisonment and all of your appeals have been denied."