If everyone agreed on exactly what "Moral duty" meant, then it would probably be a great thing to consider.... but that's unlikely to happen.
From a practical perspective, if organizations allowed their employees to voice their opinions against the organization, the chaos that would be created would result in the failure of the organization. Imagine what would happen to the stock market, if public company employees could "say whatever they wanted"? What if the government's finance department employees could "say whatever they wanted"? What if the defense department didn't "muzzle" their employees?
That's why all organizations create codes of conduct for their employees and enforce them.