I'll be honest, I had not even noticed this. Even though I had used long press many times in the last few days. It does look to only show apps you have installed, as I only had Amazon show up on the list for me.
That said... Honestly, I can see the appeal for some people to have their favorite store show up on a quick press. No need to copy, open the app and then look it up. As long as it is limited to select few pre-approved apps and not every apps can just hijack your longpress menu to add their junk.
While google added this feature, it wasn't google that enabled this for each apps. It's Amazon themselves who updated their app to have this thing in it. If you uninstall the updates, it will be gone. The apk from last october (26.21.0.100), is confirmed to not have this built-in according to a reddit comment. I tried it, it worked. But then again, you have to get access to a secure apk, maybe not worth it... This is why I hate when apps automatically update.
You could always uninstall the Amazon app, launch your browser, go to Amazon and through your browser, use the "add to home screen" feature to make a pseudo app.