The burden of proof in cases like this is actually carried by the mountains of emails, product design documents, and all other forms of internal documentation. Only the e-mails get regularly purged.

This isn't a dope dealer growing weed in his car and having his buddies sell it for him. These sorts of operations produce a lot of paperwork. As an employee who is actually tasked with building/working with all of these systems/processess, you don't get inducted into the secret conspiracy with a nod, wink, handshake, and a secret decoder ring.