In order for a shoe to be labeled as manmade by law in the US it has to be. Nothing containing natural fibers can be in it. If it says synthetic then at least some part of the shoe must be synthetic. If it says all synthetic it must be. It is kind of like food. There are people who are allergic to wool, and silk, and the chemicals used to prepare Leather. If a shoe is mislabeled the manufacturer could be sued. You only have to becareful about things that list "fabric" upper. Fabric can be made of any type of fiber.