It's the job of the union to advocate for workers, not for management. Workers want more pay, job security, benefits, safety, and so on. What employers want is entirely different and often directly opposed to what the workers want. And there is already a strong advocate for the employers interests - the employer. In most employment situation there is no advocate for the workers to begin with.
What you are suggesting is that in the rare cases that workers do have an advocate, that advocate should also be acting for the benefit of the employer. I don't think that adds up.