Achieving the betterment of labor without government coercion

In Florida Tomato Fields, a Penny Buys Progress

Florida tomato field workers toil under harsh conditions for low pay.  It seemed that the tomato growers could dictate whatever labor terms they chose, yet such was not the case.  It took many years, but these workers managed to convince the large purchasers of tomatoes, such as Walmart and McDonalds, to buy tomatoes only from growers who treated their workers better.  Nowhere in the article is government intervention mentioned.  The agreements between the workers, the growers, and the purchasers were completely voluntary.  If tomato field workers, who probably have less political power than any group of workers, can better their lot without government help, one wonders why government intervention in labor markets is needed at all.