I am at a university (Michigan) where the union is primarily for graduate students that teach. I could not tell from the writeup, but it sounds like this is for _all_ graduate students at UChicago.

Being in an engineering department, I never had an issue with having to teach a heavy load of classes. But, some of my friends in humanities departments had a teaching load of - 6+ hours of classroom time + discussions + grading long papers - for classes of 50+ students. Being part of the union allowed them to put pressure on their departments that they needed to hire other TAs so that they could focus on their research and not spend additional years writing their dissertation.

I don't think being part of a union is going to stop any motivated student from doing their own research. And part of joining the union is to ensure that their members do receive a good stipend and have their health insurance paid for while maintaining a reasonable teaching load.