Can someone explain to me why domestic US students are much cheaper than international students for public schools?

[90% Caffeine]

 Ok so I browsed through this website and it seems like there is a general consensus that public schools accept fewer international students because they don't receive financial federal aid and they cost more. I don't really understand this argument or how universities run. It might be obvious to some of you, but it's very confusing and I hope someone explains the difference to me. 

 For STEM PhD programs (mainly biology and chemistry), departments only have to consider two bills when accepting applicants: tuition cost and stipend. Stipend is fixed across all students and there should be no difference between domestic and international applicants. As for tuition costs, all STEM PhD programs WAIVE tuition costs for PhD candidates. The way I understand this is that there are no tuition charges, and so there is no sense in saying domestic students are cheaper than international students. If my understanding is valid, then where does the government/federal support fit into this situation? What is being subsidized? 

 

 Thank you for taking the time to reply!  

[ExponentialDecay]

Domestic students are eligible for in-state tuition.

Departments waive the tuition for you (i.e. you don't have to pay it), but that doesn't mean that money doesn't exchange hands inside the university. Teaching you, maintaining your lab, your office space, etc. actually costs the university money - and they have to pay it to the graduate school or whoever. 

Public universities are being subsidized by government (mostly state) money, and the justification for that support is that public universities exist to educate the residents of that state. That's why they have in-state tuition and out of state tuition, and why tuition at most publics is lower than at equivalent privates. As a non-US citizen, you're technically not eligible for that support. 

[Boolakanaka]

Yes, when the OP says that the respective state institution waives costs, he/she is conflating it with the notion that there is NO costs—when in reality those costs are still accounting for and bore by the university. 

[90% Caffeine]

Thank you for replying! Yes you two are correct. I don't know how universities run and I just assumed the department cover the cost of grad students tuition from grant funding/undergraduate students paying tuition.