There are states with statutes that protect employees when they are talking about how much money they make.
Here’s one from C.R.S:
Unless otherwise permitted by federal law, for an employer to discharge, discipline, discriminate against, coerce, intimidate, threaten, or interfere with any employee or other person because the employee inquired about, disclosed, compared, or otherwise discussed the employees wages; to require as a condition of employment nondisclosure by an employee of his or her wages; or to require an employee to sign a waiver or other document that reports to deny any employee the right to disclose his or her wage information.
Edit: this one is actually considered “discriminatory or unfair employment practices”