So that foreign nationals think it's a smart idea to move to the US and do research for us. So that when they complete their PhD they want to stay permanently and continue doing research that benefits the US. So that despite country humanity gets the smartest people together doing work that might benefit the entire world?

A full scholarship to somebody that decides to move back to their country because of racism and xenophobia still directly benefits the US if that research was done here. The smartest students in the world passing on the US does not help the US. With more policies like this the smartest students in the US might move to other countries so they can work with a larger pool.

How many promising American-born researchers are we missing out on because we give away valuable training and research opportunities to foreigners?

Don’t forget that America’s technological heyday—when Silicon Valley was built in the first place—came during and shortly after the Johnson Reed era of immigration restriction. Companies like Apple, Intel, etc., were founded between 1960-1980 (the decade on each side of when the foreign born population hit the historical low in 1970).