What about games on non amazon app stores?
Amazon sells those too. When you buy a game on Amazon, all you're really buying is an activation code for the game on whichever platform hosts it (Blizzard / Valve / Ubisoft / Origin).
That's what it's been historically, but they did recently introduced their "Twitch Launcher" app that they plan to have act as their own digital download client. They're not selling anything for it just yet, but they are currently beta testing it as a platform for giving away freebies to Twitch Prime subscribers. Once they start selling games directly on Twitch I can't imagine they'll have many Steam/Origin/Uplay codes for sale when they've got their own digital distribution platform in play.