It seemed to me like when I went to grad school that a lot of students were there for the freedom to explore their interests. They then quickly realize that their freedom is subject to a bunch of constraints like the availability of grants or the specific interests of your advisor.

And if you're going to be that constrained may as well get a job and make some money while you're doing this.

Your salary as a PhD student is a joke but you're still an expensive asset as far as your advisor is concerned because of your tuition which pays for stuff like maintaining the campus, allowing you to take classes, your office etc..

However, I feel that most of those resources are useless as far as a student is concerned. You just need to pay for rent and maintain a relatively comfortable lifestyle. So something like $2,000 a month would be more than sufficient for a student and is a tiny cost for an investor looking to have interesting research published either as a paper or open source project.

The only real working relationship in a graduate program is the relationship between faculty and student so maybe it's worth it for some of the wealthier faculty to cut out the middlemen.