Amazon Restricts How Rival Device Makers Buy Ads on Its Site

Amazon.com Inc. is limiting the ability of some competitors to promote their rival smart speakers, video doorbells and other devices on its dominant e-commerce platform, according to Amazon employees and executives at rival companies and advertising firms.

The strategy gives an edge to Amazon’s own devices, which the company regards as central to building consumer loyalty. It puts at a disadvantage an array of gadget makers such as Arlo Technologies Inc. that rely on Amazon’s site for a significant share of their sales.

The e-commerce giant routinely lets companies buy...