The system is abusive. Illegals are paid less than legal workers, Americans or H2 holders. Working conditions may or may no meet legal standards. Social costs are paid by the taxpayer. Immigration and tax laws are violated as a matter of doing "business". In my view, the blame lies largely with the employers, and law (non) enforcement, from immigration to the IRS.
BTW, a legal, H2 visa holder can travel to the US, fare usually paid by the employer, work hard, earn money, take it back to Mexico, and come back next year. All in the light of day. And in fact they do, I've posted several articles on operations using H2 workers.
Personally, I'd like to see the program require some level of medical and liability insurance required as well.
How is it abusive? The illegals voluntarily accept the lower wages. They can leave if and when they want ... but they don't. The growers can't really afford to pay much more, and still stay in business.