It's not though. Establishing Brand X as a cause doesn't tell you what ingredient, or what missing nutrient, or what other feature of that brand's food is causing the problem. What value is there in that? You can switch brands, but you have no way to know the new food isn't worse than the old food. And if you're the producer, you have no knowledge of how to improve your food - might as well just close up shop because ... the brand is the cause?