https://www.nlrb.gov/rights-we-protect/employee-rights
This comes from Section 7 of the National Labor Relations Act: "Employees shall have the right to...engage in other concerted activities for the purpose of collective bargaining or other mutual aid or protection."
In this specific case, the Advice Memo refers to protection for "expressing a dissenting view on matters affecting working conditions or offering critical feedback of its policies and programs".