Every single major retailer does this. It's not collusion, it's just smart policy. Shelf space is insanely valuable. Retailers have customers walking around their stores (or making searches in Amazon's case) at the final step in the sales funnel. No retailer wants to get a customer that close to buying (often at considerable expense) then lose them because the brand either has weak channel price control or is trying to divert customers to their own website by offering a discount.