The USDA does not grade companies, they grade individual cuts of meat. They have not graded Tyson a D, and even your own link shows that.
OSHA has graded Tyson a D, not USDA. OSHA judges a company based on workplace safety, not on food safety. It is a completely different thing.
They have always had the right to sell food directly to supermarkets, or even consumers. I can go to a farmers market and buy food directly from a farmer.
This allows farmers to process food without any food safety regulation. Processing food can introduce some extreme health risks, and they will not have to do anything about those.