They weren't sued because there was carcinogens in the chicken, but they were sued because of California's Proposition 65, which stated that if there's anything dangerous in the products then the companies had to give a clear warning.
Eles não estavam sendo processados porque havia cancerígenos no frango, mas foram processados pela Proposição 65 da Califórnia, que diz que se há algo perigoso nos produtos as companhias deveriam dar um anuncio claro.