The current situation is as follows: to work in most of the government research laboratories, particularly in the computer science and technology areas (the US Naval research lab, Sandia national labs, NSA, etc. to name some labs), the applicant needs to be an US citizen. However, more than 70% of the PhD students in the top 25 universities in the US are not US citizens.
So, there is a lot of lost cause by disallowing this big chunk of bright PhD students from helping the research at these government labs. Most of these international students would eventually become citizens of this country, and it seems like a good idea to let them work at the government research labs as soon as they get their PhDs (perhaps after a comprehensive background check).