A grad assistant is usually not perceived as an employer by most universities, US and abroad. As such, from the university's perspective (which you took in your comment), one cannot say that they are being compensated, at all.
Assuming that you do understand that a graduate assistant (in this case, e.g. a research assistant who is also a student) is actually an employee of the university,
- The training they receive (courses et al) is what we call unpaid training. They do not receive salary for the hours they spend receiving training, which is a requirement for them to do their job (research assistant) efficiently and successfully.
- Would you accept payment in prestige if you were in, say, Google or Apple? A graduate school has much less prestige (hence the need for "internships").
- Independence does not exist for a grad assistant, whose not only livelihood but also school registration depends on her/his relationship with her/his "mentor" (i.e. boss / advisor to boss) and other department members. Not only job security but also school security depends on maintaining "good relationships" with these people. (As a student, on the other hand, s/he is expected to produce and engage in independent thought.) Additionally, this particular kind of employee cannot switch jobs easily since most jobs will not provide benefits for external education.
- No or lower-end benefits etc.
You can very easily hire a grad student on a "free market"[1] for even $15k/yr if they are [made to be] desperate enough. Most already are.
[1] Please read Adam Smith to understand how a "free market" actually operates. It's not what you think it is.