Michigan does have law addressing it
Michigan: Mich. Comp. Laws Section 408.483a
408.483a Prohibited conduct.
Sec. 13a. (1) An employer shall not do any of the following:
(a) Require as a condition of employment nondisclosure by an employee of his or her wages.
(b) Require an employee to sign a waiver or other document which purports to deny an employee the right to disclose his or her wages.
(c) Discharge, formally discipline, or otherwise discriminate against for job advancement an employee who discloses his or her wages.
This provision was added to Act 390 of 1978, Payment of Wages and Fringe Benefits, by Act 524 of 1982, effective March 30, 1983.