Because when I admit a student to my lab, I'm talking about spending five years helping someone grow as a future colleague, and, in the process, advance the science produced in my lab.

"Cheap labor churning out publications" doesn't support those goals.

I also was a graduate student once, and am endowed with basic human empathy.

Because they care about their students, but can't pay them any more than the university allows.

I think you are overly cynical. To me it looks like many professors view their grad students more like a protege than a minion. As such while the greater system is perverse, and the professor looks out for themselves, many professors would still have their students interests in mind.

Perhaps you are right. My wording may have also been too strong. But I still think that unions are not in the best interests of the faculty.

Well, of course they are not in the best interest of faculty. The point is at least some of the faculty is perfectly human enough to advocate policy that is not necessarily in their own best interest, for the betterment of others they care about.

The university pays for the graduate student salary, not the faculty.

Edit: My knowledge of funding comes from a sample size of 1 (wife's cousin doing genetic engineering out of St Louis) so it's possible I've made a mistake extrapolating too widely.

I'm a grad student myself, so I'm speaking from experience.

As far as I know, in virtually all US public universities, faculty members have to pay for research assistants from their own grant money. The university I am at helps the faculty by charging less for the student's tuition (out-of-state to in-state).

Maybe you are confusing RA funding with TA funding?

It depends on the universities. Yeah for most public universities, the grad students are directly funded by the professor's research. In many other universities, grad students are funded through the department, as in they are guaranteed a salary; the salary would come from the professor (or other fellowships the student got themselves) if they a research assistant, and it would come from the university if they are a teaching assistant. They are guaranteed funding because if the student doesn't get research funding, then the department sticks them in a TA position so that they get a salary.

Isn't it mainly TAs who want unionization? At least in the physical sciences, RAs are indeed paid with grant money.

My graduate students are funded directly from my startup funding and grants.