Can post graduate students be forbidden to work in the institution they are studying? I have seen some graduate (PhD and Masters) students in Turkey, Nigeria, Malaysia etc that can not work in the university they study. In fact I am one of them as I am about to finish Msc mathematics and computer science in Turkey and planning to start PhD in USA. My question is, is it a policy of most universities in the world that post graduate students can not work in the institution they are studying and earn their living? Even to work as graduate assistant perhaps.
No.

In North America, at least in science and engineering, PhD students are generally expected to work for their host universities as either teaching assistants or research assistants, except for the relative few who win full-ride fellowships.  This expectation is strong enough that it is considered foolish to accept an offer of admission to any PhD program without a simultaneous offer of a fellowship or assistantship.  Sometimes these assistantships are also available to Master's students.