Why would Walmart allow Kelloggs to advertise their products availability for less in a different store, on Walmart's shelves? Or Kelloggs saying that you can p

Why would Walmart allow Kelloggs to advertise their products availability for less in a different store, on Walmart's shelves? Or Kelloggs saying that you can pay them direct and the just help yourself to their product from Walmart? It undermines the whole premise of commerce. If you want direct sales (sideloading, which should be optional), pay for the infrastructure. Don't expect to sell rent free on someone elses...