My own view on this issue is that it is equitable and fair so long as the workers have the choice of whether to join the union. It is very hard to say that a provision that a worker voluntarily chooses to submit to could possibly be inequitable or unfair. If it were, the worker would, presumably, choose to work in a firm that is not unionized or would choose not to join the union.
Now, let us simply ask whether unions should make this tradeoff. I would argue that, in many professions, it is a good trade. Unionization works best in industries where one worker is not likely to provide a lot more value to the firm than the next worker. For example, airplane pilots are unionized. It makes much more sense for them to all earn cost of living raises because presumably one pilot does not add more value to the firm than another. As long as the pilot is flying in a safe manner, the passengers are not going to prefer one pilot to another. By contrast, professions like the law are not typically unionized. This is because some lawyers in a firm bring in more business and are therefore more valuable to the firm. Since most unions are in industries like the airlines where workers are fairly interchangeable, it makes sense for them to push for equal pay raises for all, in my opinion.