They don’t pay for students’ own research for their final projects; those in teaching or administrative assistantships get paid the same as researchers because the stipend is paid for the labor and studies for the final projects are on their own. But plenty of times the researchers choose (or I’m guessing are pushed into, I never signed up for a research one) a topic a professor is already working on to do their own project’s research and that’s where the line that’s clear for teaching and administrative assistants blurs for research assistants.