If I were Blue Buffalo I would never say never. Large manufacturing plants always have the chance of having by-products in their packages. That's why so many companies put warnings like "may contain soy or peanuts or dairy".

I doubt Blue Buffalo would make those claims unless they really had less by-product in their packages, but it's the "never" that will really get them in trouble if consumers or other companies decide to pursue legal action. It doesn't even appear that Purina found that much and they wouldn't have to publicly release the results from a third party to be considered remotely reliable to me.