Retailers don't exist to serve vendors. Retailers and vendors exist to serve consumers.
I didn't claim that retailers exist to serve vendors. But I also don't expect retailers to have the power to destroy vendors single-handedly. And remember that a large part of what's going on here is an elimination of the customer's feedback through Wal-Mart's buyers. That's exactly the problem with a lack of competition -- the retailer replaces the consumer with respect to calling all the shots and retains all of the power.
Let me put it this way. Do you think monopolies are harmless? If you don't, why aren't they harmless?