http://www.theatlantic.com/magazine/archive/1995/11/in-the-s...
>>> Migrants are among the poorest workers in the United States. The average migrant worker is a twenty-eight-year-old male, born in Mexico, who earns about $5,000 a year for twenty-five weeks of farm work. His life expectancy is forty-nine years. <<<
I'm hoping some things changed since 1995, but as someone used to to construction in California and had to compete against dry-wallers earning $3/hour, I am doubtful.
One would have to be very careful in choosing "ethical food," lest they have tunnel vision in only protecting animals, and by their pure nature of replacing lost calories with more vegetables, they encourage more human slavery in the world, and yes... in America.