Here is an excerpt; Read the whole thing here, and also see the extensive symposium on informal restrictions on free speech:
Both federal and state laws include protections, including some tort common law, for speech that advances public policy or the public interest. A number of laws protect employees from blowing the whistle, or disclosing illegal or harmful conduct, or asserting other employee rights—for example, reporting discrimination or alleging violations of wage and hour laws…. [A]Half of the states protect some form of political speech or association by private sector employees (especially those that are off-duty and closely related to the electoral process). The number and breadth of those laws on their face is astonishing; As yet they have not made many cases, because they are not well known and are more or less bound by respect for the employer’s interests and privileges.
Importantly, all of these private employee speech protections … take the form of exceptions to the employment background rule—that is, the employer’s privilege, absent an agreement to provide job security, to terminate employment at any time and for any reason or no reason, although not for any such reason. which is specifically prohibited by law. There are many such prohibited grounds, or exceptions to wrongful discharge in employment, including just reviewed speech protections as well as the large and still growing body of employment discrimination laws.
But employment background rules undermine each of those protections. Employees who suspect or believe that their discharge was actually unfair under the law of the land must first identify the wrongful motive (which the employer usually tries to conceal) in order to determine in which forum to file a claim. The burden then falls on the employee to navigate the obstacle course of trial or litigation, and to prove an unlawful intent on the part of the employer, who controls almost all the relevant documents and hires most of the witnesses….
Private sector workplaces have many quasi-First Amendment loopholes; But filling that gap would do far less to protect workers’ free speech than to protect them against arbitrary, unfair dismissal along with the rest of the world—that is, providing the necessary backstop to due process rights for any employee’s speech rights. Due process rights in the form of procedural and substantive protections against unfair dismissal will place the burden on the employer to show a valid and sufficient business-related reason for the dismissal. Legally protected speech (such as union organized activity or off-duty political speech and association) will not count as just cause, and the wrongful-dismissal review process may provide an accessible forum to air an employee’s claim that the employer’s true cause or dismissal The purpose was illegal. Beyond that, however, requiring the employer to show good cause for dismissal provides a buffer of protection against discrimination or retaliation, and without the doctrinal difficulties of defining the precise boundaries of what is protected or the often fatal difficulties of proving the employer’s intent… .
Formal free speech rights that about half the states carry in some form … would be far more secure in a just world, yet subject to reasonable employer regulation. And employees in the other half of the state would gain a measure of indirect protection for their political speech and association—especially for their off-duty activities—as an incidental byproduct of the employer’s burden to prove a good and sufficient reason for dismissal….