Let me explain. There are different reasons why foreign workers benefit all of which comes down to one simple thing. Money
Large Outsourcing Companies (Infosys, TCS, Cognizant, IBM, Deloitte, Accenture etc)-
This is simple. They can replace most of the workers and ship at least a part of the jobs back to India (let's face it, most IT jobs are outsourced to India). For example, they laid off about 300 people. The rates charged to the client are usually about $50-$90/hr for people onsite and about $16-$40 for offshore. Let us also assume that the average salary for those 300 workers is, very conservatively, about $70,000/p.a. So for about 300 people that works out to about $21 Million. Over three years that works out to about $63 million just in salaries alone. This does not include their insurance or other benefits. When these large companies take over, they get what is known as Fixed Price contracts. So Disney would say, I will pay you $40 million, over the next 3 years. The company can get employees for very cheap and they are young. If you get a chance, go to any corporate IT office and look at the Indian employees. Their average age would be less than 30. So you have young people, who see an opportunity to earn thrice what they would in India.
Smaller contracting companies -
These guys get fresh college graduates, mostly Indians and Chinese, pad their resumes and market them out. If you compare costs, they would be the same cost to the client, however, most of these companies are MBE which allows companies that contract out to them get significant tax breaks. Not to mention no costs for benefits and ability to hire and fire far more easily.
Again, this is not all of the foreign workers, but about half of them that utilize H1b. The other half are genuinely good, capable people who really have skills that are not easily available here.
This also explains why companies keep asking for more H1b visa which is the primary migrant tech worker program. You get employees that you can get rid of anytime, can pay substantially less wages and they are essentially indentured to you. Which corporation would not want that?