Why do minorities do the job for less while others demand more?

Many employers, at some point in their business practices, are faced with a decision on whom they should hire.  The first choice is hiring the traditional employees that will be paid competitive wages, plus additional money will be allocated to their benefit plans.  The second choice is hiring imigrant workers that will do the same job but, will do the job for a smaller paycheck and no additional benefits that costs the employer even more money (Parrillo 2011. p. 150).  The obvious choice seems to be hiring minorities to do the job for cheaper so the employer will spend less to make the same amount, giving him a larger wallet at the end of the day.  How is this possible?  Why do minorities do the job for less while others demand more?