> They have to do it, whether they are on a stipend or not.

This is like saying that undergraduate students are unpaid for doing their homework.

It's part of their education to learn how to take part in the community by doing things like reviewing papers.

Well sure, but nobody benefits financially or reputationally from an undergraduate student's homework. There's clearly some proportion of having an academic's PhD students do this type of work for them which would stop being educational and start being an abuse. The question is, is too much of this type of work being pawned off on grad students?

> It's part of their education to learn how to take part in the community by doing things like reviewing papers.

And this "part of their education to learn" keeps really cheap labor running science. Even postdocs are considered trainees, because they will always be learning because is part of their "job"

> A lot of reviewing for conferences is actually being done by Ph.D. students. They have to do it, whether they are on a stipend or not.

Most (all?) PhD students in science and engineering are on fellowships, at least in the US. Major research universities will not admit unfunded PhD students.

Oh boy, would I love that to be the case as an unfunded PhD student, working a separate job as visiting faculty.