[throwawayaway12]

I think it is important to emphasize that 'no tuition' is the norm for STEM fields, and not necessarily for non-STEM.

I will rephrase the scenario slightly: The university collects tuition on PhD students to recoup costs in education. This can be for paying professors (who teach the courses and advise the students) to paying electricity the student uses when doing their research. Getting a PhD is a questionable decision in terms of finance for the student. Advisors are able to write into grants support to pay tuition for students. This makes it easier to entice talented students to work with said advisor and get a PhD, rather than joining the workforce straight out of undergrad.