Hmm. Even if TAs don't assign grades for a course, if they refuse to grade the assignments that can make it extremely difficult for the professor to give the students accurate grades.
I think "just" is underselling it a little. When we would grade exams it was quite an ordeal. The professor and all the IAs (maybe 5-8) would be in a room and grade all day long. We would order food delivered to the room so we could keep grading straight through meals.
Having 1 person do all that work would be intimidating.
