In science and engineering, PhD students are typically paid a salary and have their tuition waived. This is mostly funded by the federal government though the NSF, DOE, DOD, etc.

It's the same in Germany. Getting a PhD is basically a job.

In the US, getting a PhD is not a job. Everything below the faculty level is considered trainee.

I thought this applied to the humanities, arts, and social sciences too.

I think the stipend is much lower but yes. It also depends on the university how much the stipend is. At some schools it might only be like 15k which is not enough to live so you would have to find some way to work also.

This isn't exactly the case.

Many training funding mechanisms (ie. NIH Training Grants, Fellowship awards, & Career Grants/"K-awards") are not allowed to fund non-residents [1]. Funding international students is a difficult problem for most PhD programs in the life sciences. I don't have direct experience in physical/social sciences, but I believe similar restrictions apply to NSF funds.

I believe most PhD programs cover international students using a mixture of direct project funds (ie. an NIH R01, the same grant that covers reagents/equipment), private fellowships, and endowment money.

1 -- http://www.niaid.nih.gov/researchfunding/sop/pages/foreignwo...

Research Assistant, Teaching Assistant. Thats usually how. (its not really for free)