The First Amendment of the USA Constitution provides free speech to us all. There are limitations, of course, however.
An employer is only restricted in its communication to a potential or new employer or anyone else IF there is a written contract (example: severance agreement, settlement agreement, etc.) in which the employer agreed to provided limited information (dates/salary, etc.) to prospective employers.
Also, if a former employer (or anyone) communicates to a third party "knowingly false" information AND the former employee is damaged by the falsehood (gets fired, etc.,) the employee may have a case of defamation. The person claiming defamation has to prove that something was communicated about him/her that was knowingly false.
Many employers have a policy in which they provide only limited information to prospective employers because they have been advised by their legal counsel to do so to avoid lawsuits based on defamation and other grounds. However, a company's policy does not create a law and the employer can still say whatever they want to say since they enjoy free speech.
If you are an employer and would like consultation about company policies or laws regarding labor and employment law or if you are an employee or former employee with employment law questions or issues, call Law Office of Kristine S. Karila. (949) 481-6909. Ms. Karila has been practicing law in California since 1992 and specializes in labor and employment law.