I am not a lawyer but it strikes me that such a threat (of action which the employer is not legally entitled to take) would probably violate a law or two. This wouldn't help the employee unless they went to court or the NLRB with it, but if some employee eventually did the court/NLRB may require things from the employer to prevent such incidents in the future.
You describe an enforcement, not legal issue. Even with this legislation, the manager can still verbally threaten the employee.