Because if done properly, employees interests are aligned with the interests of the employer. If you own significant stock options in Facebook, it's not very w

Because if done properly, employees interests are aligned with the interests of the employer. If you own significant stock options in Facebook, it's not very wise to to cut the branch you are sitting on. Additionally, external researchers have much higher risk, because they may seek fame, or need money (employee are generally in a safe position, the intern at a research group is not). An exclusive data leak can bring both advantages. Also, they may not have the same security practices as internal ones (deep packet inspection, etc).