Farms usually blame "large corporations" for not paying them properly, so they're "forced" to pay poor wages.
That's why CIW made these bargains directly with the large corps. To stop farms from passing the buck and playing "Aw shucks, I'd love to pay fair wages but I can't afford to."
And you know what? It took time to get the big food corps to the table. But most of them ultimately signed on. In the end, they were a lot more amenable to proper farm wages than most of the farms.