That's an interesting example. While undoubtedly large, Walmart is still a brick and mortar shop, it's part of the short tail world. Walmart can press producers

That's an interesting example. While undoubtedly large, Walmart is still a brick and mortar shop, it's part of the short tail world. Walmart can press producers for significant discounts by simply affording them some of it's limited shelf space. Amazon is "the everything store", the exponent of long tail online retail. If Amazon would employ the same tactics as Walmart, it would be quickly be out-competed by other online stores with a larger selection of products.