>This is common practice among tenured professors with grant money to spend and graduate students beholden to them (I’m not making a moral statement, just noting this is common practice).

No, that's not common at all, nor how the system works. Use of graders/TAs are on an institution basis.

"Grant money" has absolutely nothing to do with it. Grant money is earmarked for specific research and cannot be spent for teaching purposes. When grant money is used to hire graduate students, they are research assistants and must work on the research project which the grant money was given for.

TAs/graders come from separate tuition budgets. Tuition dollars are much more preferred anyway because they're stable and faculty do not have to work for them, they show up based on enrollment.

The fact you have TAs/graders at all is by institution. Large R1 institutions have grad students and undergraduate population to fill these roles. In these institutions, 1 minimally sized undergrad class = at least 1 TA, with additional TAs and graders after some enrollment threshold.

Tenure has absolutely nothing to do with it. Adjunct faculty get TAs/graders all the same.

Indeed, to support your point, I had a teaching assistantship for my first two years of grad school, meaning that I worked on an as-needed basis for things like supervising labs, proctoring exams, grading papers, etc. It was funded by the department. Where I went to school, the TA workload was pretty light.

Then I got a research assistantship, which was funded by my advisor's grant. I meant that I got paid to do my thesis research, and no longer had to teach. This was a precondition for joining his group.