They aren't obligated to represent all employees equally in collective bargaining. It'd be impossible for them to do so, because there are intra-employee confli

They aren't obligated to represent all employees equally in collective bargaining. It'd be impossible for them to do so, because there are intra-employee conflicts of interest. Rather than being a fiduciary institution, at least when it comes to negotiation, it's a democratic one that only needs to represent the interests of 50% + 1 members of the union. Often this manifests in trading off the interests of newer members in order to secure benefits for senior members -- sometimes even going so far as to bargain on behalf of retirees who aren't even members. But other configurations of favored and non-favored groups exist (another fairly common one is part time or seasonal employees paying dues but not being effectively representated during contract negotiations). Grievance representation is a different matter, there the union is much closer to the fiduciary ideal.