I took a preliminary look at the entire lawsuit, and Purina's claims are based on a very small number of samples analyzed by an unnamed "independent testing laboratory." The byproduct meal was detected in some products, and not in others. The grains in the "grain-free" products were detected at very low levels in some products and not in others. The suit doesn't name the testing laboratory, nor does it provide detailed results or say how the tests were done. That information will probably come out in court. The problem with Blue Buffalo's advertising is that it uses terms like "always" and "never," so if Purina can find one bag of food with a bad ingredient, then they can say that Blue Buffalo was using false advertising. We'll see how this all plays out. Here is a link to the lawsuit:
Pet Food Honesty | The Complaint