> It's really an employee-employer relationship with on the job training.

I don't think so. When I was a Ph.D. student, I spent most of the time doing my own thing (learning and research), a small portion of my time was spent on working. I imagine this could be different by field, and I know some other departments pay way higher stipend than my department.

I think the lowest paid graduate students belong to social sciences and humanities, but their "work" as graduate students also has lower value compared to other fields. They are also the ones with strongest push for unionization, because they (maybe for good reason) are the lowest paid and because their ideology.