In hindsight, the pitch does seem a little dubious: Let Amazon put delivery lockers in your stores, and people coming in to pick up their Amazon orders will probably buy something else at the checkout counter before they leave.

Apparently, that wasn't working out as planned for Staples or RadioShack, which are reportedly giving the boot to Amazon's bid for brick-and-mortar presence.

The decision by both companies to end the Amazon Locker test run was first reported today by Bloomberg, though neither was very specific about why they kicked Amazon out.

RadioShack told WIRED its test of the lockers actually ended months ago. Much of RadioShack's senior leadership is new, said spokeswoman Merianne Roth, and the lockers "didn't match our move-forward strategy." Staples would only say it had been testing the lockers: "We evaluate many new products and services in our stores and this was another example." Amazon offered a generic statement praising the convenience of its lockers.

Still, the reasons Staples and RadioShack would come to such a decision aren't hard to fathom.

Office supplies and electronics are both commodity businesses. Retailers in those categories aren't typically competing on product. They're all more or less selling the same thing. As a result, stores must rely on other factors to differentiate themselves, such as marketing, convenience, and price.

Amazon has become the world's largest online retailer – and one of the largest retailers in the world, period – because of its mastery of efficiencies, from ordering to inventory management to delivery. That mastery has allowed Amazon to undercut competitors in nearly every category of merchandise on price. For Staples and RadioShack both, the specter of Amazon must haunt their C-suites daily.

By bringing Amazon's lockers into their stores, they seemed to be subscribing to the "keep your enemies closer" strategy. But it's easy to imagine people walking into either store, picking up their Amazon orders of office supplies or smartphones, and walking back out without a glance at anything on the shelves.

By adding the lockers, both chains were helping to augment the convenience of shopping with Amazon. When the ability to walk into a store and back out with the product you want is one of the few key advantages brick-and-mortar stores still have, the decision to give Amazon a similar advantage makes little sense, especially as Amazon marches toward same-day delivery.

The decision by Amazon to dramatically build out its warehouse network in order to saturate the most populated urban areas with its presence portends yet more pain for brick-and-mortar retailers of all kinds. Most recently, Amazon's move into groceries puts it in direct competition in one of the few categories where it doesn't dominate.

For now, the presence of Amazon's lockers in 7-Elevens seems safe. You're probably more likely to order a phone from Amazon than milk or a pack of gum. But if Amazon has its way, you won't think that way for long. In which case, Amazon may have to start looking at buying up some of its own brick-and-mortar real estate if it wants its lockers to have a home.