For what it is worth, McDonald's says on their web site that this is not true. So either you are mistaken or McDonald's is committing fraud. Do you have a source for your claim?
From the McD's web site ( https://www.mcdonalds.com/us/en-us/about-our-food/our-food-y... ):
"Every one of our burgers is made with 100% pure beef and cooked and prepared with salt, pepper and nothing else—no fillers, no additives, no preservatives."
"Do you use so-called 'pink slime' in your burgers or beef treated with ammonia?"
"Nope. Our beef patties are made from 100% pure beef. Nothing else is added. No fillers, no additives and no preservatives.
"Some consumers may be familiar with the practice of using lean, finely textured beef sometimes treated with ammonia, which is referred to by some as “pink slime.” We do not use this. "