This is a common misunderstanding. For example, an employer can propose a wage cut (obviously not a concession) as part of good faith bargaining. To your point

This is a common misunderstanding. For example, an employer can propose a wage cut (obviously not a concession) as part of good faith bargaining. To your point, making concessions will often prevent any worker (or the union as a body) from filing an Unfair Labor Practice charge. But employers with good counsel seldom worry about such charges, because the penalties are so weak.