Yeah, you're right. Obviously, most employees may not have the financial resources to do something big enough to be scary, but this may add up : if a company is public, it probably has a huge employee base. Employee may even not realize they are doing harm.

What would you think about allowing insider trading only on good news, that is, only buy on insider news? The only effect it would have would be miss opportunity for potential longers, but it would leave no one bag holding.