How come people don't sell the meat of an animal, that died of natural causes?

If the animal dies before slaughter they destroy it they don't sell it because if it made people sick they could go to jail and lose everything they own. The USDA watches for these types of abuses but it just would not be ethical for them to do this. Lets say it died from some form of Cancer it is possible to pass this on to humans so when butchers are cutting up an Animal if they see something bad they dispose of it and report it to the USDA. The USDA inspects the meat and grades them they are also tested for chemical additives that could be harmful. You can look it up at the USDA web site and wikipedia as well for more information.