I think there’s more responsibility when a firm has a contract with a supplier to manufacture their products compared to buying a product or an input. In the first case there’s some research or inspection of the production process, materials, quality control, if they can comply with delivery times etc. Unlike, if they were just buying batteries or bearings, for example, where the only inspect the final product.
Btw, I’m not saying Shimano was aware of this but they do have responsibility, at least enough to investigate it. Also, they have a plant in the same region, so they know the local labor conditions.