An example of a specific labor standard you'd like to see implemented would be welcome.
Also just to be precise, tuition isn't just to pay for classes, it goes to pay for all of the resources you use as part of the university community. If you are done your requirements you will still have to pay tuition. And students don't stop taking classes by the end of year 2 (or 4 depending), they stop taking required classes. Since your advisor pays your tuition, you can negotiate with them if you want to take more than the minimum. You are done your requirements after 2-4 years and don't have to take more classes, but of course you can (I did). Maybe this is something unions can help with.
My indulgence for "I spent three months going down this rabbit hole because it might have proved interesting" is way hired for graduate students than it would be for research staff, because that's part of them being students.
Universities are rather analogous to sweatshops in how they operate and the economic reasons for doing so. And I suspect that, in much the same way, this state of affairs represents a sort of Nash equilibrium that cannot be significantly altered except by an outside force changing the rules of the game.