I was working in technology in Oregon recently. Oregon has a vast farm industry. One lady who worked in a different department told me resentfully how her kids could no longer get hired to do extra harvesting work because tons of foreigners showed up, driving down the wages and sucking up the positions.
Check this:
Foreign labor certification programs permit U.S. employers to hire foreign workers on a temporary or permanent basis to fill jobs essential to the U.S. economy. Certification may be obtained in cases where it can be demonstrated that there are insufficient qualified U.S. workers available and willing to perform the work at wages that meet or exceed the prevailing wage paid for that occupation in the area of intended employment.
http://www.doleta.gov/business/gw/guestwkr/
In other words, they are supposed to hire American workers first, dang-it!
Yeah, but Americans won't live 20 to a trailer (paying $20/wk) at less than minimum wage with no benefits. Therefore, they can't get Americans to take these jobs, so the employer only hires those who will. After all, his competitors are doing it!