H1Bs are killing the pipeline of American born students into graduate school.
Just as excessive offshore manufacturing hollowed out the knowledge and capability of American businesses in exchange for short-term profits, abuse of the H1B program hollows out the technical know-how of the native citizenry.
H1B is ok in concept--that is, that you bring in high-level talent that is hard to find or unavailable at home. In practice, the H1B salary is set rather low in light of the fact that H1B workers are trapped in their jobs for several years (four I think?) until they can get a green card. During that time, an employer can work them for 75 or 80 hours a week.
An American graduate student entering the workforce, perhaps loaded with debt, has to compete in that environment, often with managers above him who are more sympathetic to their Indian or Chinese compatriots. It's a block to entry.
After several years of experience, that block is no longer there, because experienced STEM people get paid better than the H1B wage, which is set around the entry graduate wage.