Free speech isn't uniformly protected in all workplace environments. Certain employees may have their speech muzzled to some extent; for instance, government employees such as teachers, police and members of the military. Military personnel, for example, can't denigratethe president and Congress according to the United States Code of Military Justice, or UCMJ.
Police officers may speak out on a matter of "public concern," although such speech might be limited if it would lead to disruption in the workplace. And teachers and administrators in public schools have to ensure students have a safe, orderly environment that's conducive to learning. For instance, a teacher could write a letter to the editor complaining about a school's lax spending policies, as that would be a matter of public interest. But if that teacher wrote a letter saying she had been unfairly targeted by the principal, the school district would be within its rights to react. In general, however, the default position is to allow free speech [sources: Center for Public Education, Policinski, Ryan].