Is it illegal to threaten an employee to not post anything negative about a company online?

Anonymous

If an employer emails their employee telling them that if they write a negative review of the company, that they (the employee) better watch their back, is that illegal? If so, what can the police do about it?

*There is no HR department at the job.

Is it illegal to threaten an employee to not post anything negative about a company online?
42 Opinion