[unholyalliance]

First off you have supply and demand, which is enough to say that new workers would depress wages. You can go deeper into the micro-economics view point and look at the utility curves of the American Engineer vs. an Immigrant Engineer, and see that they are very different. A immigrant engineer, is facing possible deportation to a country with lower wages, whereas an American Engineer is more willing to hold out for a higher wage, knowing the state of the labor market and that they do not have as pressing of a need for a job immediately. To an immigrant worker, turning down the job could lead to a much worse outcome, so their logical choice is to take the initial pay offered.

Furthermore, the business is aware of both their cost of the H1NB and of the immigrants situation, which gives them a negotiating advantage. They can use the H1NB cost as a negotiation tactic, and pay the worker a lower wage, without fear of being turned down.