The way I understand it when H-1B's are brought into the country it takes away jobs from college graduates. They graduate after 4 years thinking they will a job in the sciences and find they are all mostly going to H-1B's allowed in the country.
Here are a few more examples of that claim:
- Are Foreign Workers and Foreign Students Stealing American Jobs?
- Foreign competitors in the next cubicle
However, I have also heard claims to the contrary. For example, this fact sheet: Debunking the myth that immigration harms America.
The main piece of academic research that is cited on this subject is Immigration and American Jobs, by Madeline Zavodny, which claims that hiring H1B workers results in more jobs being created for native US workers. However, this work has been criticized for being highly sensitive to the time period under consideration (and for other reasons).
Another paper, The Supply Side of Innovation: H-1B Visa Reforms and US Ethnic Invention by William R. Kerr and William F. Lincoln, looks at patent trends and concludes that "natives are not likely being crowded-out in large numbers by higher H-1B admissions".
A more recent (February 2016) paper, The Effects of High-Skilled Immigration Policy on Firms: Evidence from Visa Lotteries by Kirk Doran, Alexander Gelber, and Adam Isen, makes the opposite claim: that "H-1Bs appear to crowd out similar workers". However, this paper is limited to small and medium-sized firms, while most H-1B visas go to outsourcing companies and large firms, where the effect may be different (e.g. the outsourcing companies' employees may not be perfect substitutes for U.S. workers, the large firms may be better placed to build up additional U.S. jobs around foreign workers, etc.)
Does the H-1B visa program cause a net decrease in the employment prospects of skilled U.S. job applicants in STEM fields?