Section 7: "Employees shall have the right to self-organization, to form, join, or bargain collectively through representation of their own choosing, and to engage in other concerted activities for the purpose of collective bargaining..."
Section 8(a): "It shall be an unfair labor practice for an employer . . . to interfere with, restrain, or coerce employees in the exercise of the rights guaranteed in Section 7..."
From https://www.ueunion.org/org_rights.html
Looks like that would be illegal to my non-lawyer interpretation.
Worse happens every day, this would be extremely evident though.