While most foreign students who attend American colleges may return home when their education is complete, some of them don't. Some of them stay here and, from a purely economic perspective, we should want to see them stay.
Trump doesn't want them to stay and may not even want to see them come here in the first place.
Student visas allow foreign students to remain here as long as they're going to school and/or working to pay for school, but the Trump regime wants to impose new limits on student visas that would automatically kick students out of the country after a fixed amount of time even if they haven't completed their education yet.
The Trump administration published notice on Wednesday that it intends to propose a new rule in fall 2019 establishing a maximum period of authorized stay for international students and other holders of certain nonimmigrant visas.
The government says the planned rule is "intended to decrease the incidence of nonimmigrant student overstays and improve the integrity of the nonimmigrant student visa." Advocates for international exchange are worried, however, that the introduction of such a rule could limit flexibility for international students and scholars and undercut efforts by U.S. universities to recruit them. The number of international students in the U.S. declined in the 2017-18 academic year after years of steady growth.
By saying the rule is intended to "improve the integrity" of student visas, they're implying that the integrity of student visas is in question.
Their integrity is only in question if you're a xenophobic racist whose only purpose in life is to identify undesirables. Basically if you're Stephen Miller.
Short of Trump's trade war, there may be nothing else more economically self-defeating than trying to kick foreign students out of the country. If they don't stay here, or if they never come here to begin with because our visa policy is too restrictive, they will take their education and talents to other countries and those countries will gain a competitive advantage.
Of course, the white nationalist brain trust in the Trump regime doesn't think that way. They don't even believe foreign students are capable of making great discoveries or founding the next big startup.
There's a fairly high chance that this proposal will be shelved after members of Congress receive an earful from the institutions in their home districts. If it's not, I expect a large number of states would challenge it in court because kicking foreign students out of the country is as much a threat to state economies as it is the national economy.