Very good point, few people notice it.
US universities are financing large majority science/engineering of international grad-school students. They expect some research and of course they expect that student will build a career and add to the name of the school.
So far it has worked pretty well. The money invested was returned in research grants so universities continue to accept international students.
Only a small percentage of these students can go to academia, few positions. If the rest are blocked by visa limitations to work in U.S., the country loses. They have trained a person for 2-5 years, spending 50k average for tuition, advisors, professors, equipment. Probably the expense is well over 100k.
Oppportunity cost is even larger (already mentioned here), many of these students are looking to stay in the States as a first option.
A lot of politicians understand these things, but this is not a time to take the right position. This is a time to be careful to your voters and try to "keep jobs in the country". So there will be some short-term satisfaction. In about 5 years, you will see the real loss of this policy ...
