The average cost of educating a PhD graduate is on the order 50 to 100 thousand dollars with a full tuition waiver; while considering the salaries a PhD student receives, it is roughly a half million dollar investment by the government to educate a graduate student.
It is not just morally wrong to send foreigners earning a PhD in the US back to their country of origin, but also economically a wrong decision.
Please find a way to provide a path to either an immigrant status (or citizenship) for people who have worked hard to received a PhD degree in the US. The country can not afford to let its intellectuals and scientists leave, many of whom really want to stay. Foreign scientists have always helped to make this country greater and safer.