Really, not because they are cheaper and wear handcuffs to their desk so-to-say since they'd have to go back home if they lose their job?
The prevailing wage rate is defined as the average wage paid to similarly employed workers in a specific occupation in the area of intended employment.
The posted prevailing wage for an EE with 5 years in Santa Clara county, for example, is about $70k (or at least it was back in 2009); so if H1Bs are paid more than that is compliant with the law.
Where's your data on that? Depending on the company I've been at the public H1B numbers were lower than what I would expect.