In blatant violation of the law:
"Employers must attest to the Department of Labor that they will pay wages to the H-1B workers that are at least equal to the actual wage paid by the employer to other workers with similar experience and qualifications for the job in question, or the prevailing wage for the occupation in the area of intended employment – whichever is greater." - http://www.dol.gov/whd/immigration/h1b.htm
The problem is that there are lot of legal loopholes in the prevailing wage requirement.
http://heather.cs.ucdavis.edu/Archive/CohenAndGrigsbyPrevailingWage.txt
I have more info tucked away some where.
In addition guest workers often pay no taxes or taxes on only a small portion of their income.