At most US universities, graduate students who are serving as employees (ie, have stipends and perhaps assigned work in addition to coursework such as teaching)

At most US universities, graduate students who are serving as employees (ie, have stipends and perhaps assigned work in addition to coursework such as teaching) generally have tuition waivers too and don't pay tuition. So your formula about tuition vs compensation doesn't really apply in the individual case. Anyway, depending on when you were a graduate student, it may currently be a lot harder than it used to be to survive financially as a grad student. Also wanting things like reasonable health insurance and a modicum of days to take off on vacation or sick leave seem reasonable to me. This is not something all grad students at all universities/departments have. In general, at many large universities, graduate students do most of the teaching (in terms of hours spent on instruction) -- what is bringing in a substantial amount of the university's revenue. And get treated terribly, at the whims of professors, in very financially precarious situations. It does not seem unreasonable to me to organize to try to get a more secure situation. If you look at the actual changes in university budgets, the amount spent on compensating those doing the teaching is a smaller and smaller portion over time, while the amount spent on administrators is higher and higher. So it might be just as accurate to consider money spent on compensating grad students as competing with administrator salaries as with tuition.