pretty much this. amazon can at any point shut down any of these services or nerf it. if you built everything up to this point on amazon and they shut it down, it's more work.
Amazon Webstore wasn't part of AWS, though. It was part of their commerce wing - very different.
remember PiCloud? they shut down after I spent a quarter building around their API calls. I never want to repeat that mistake and you also get the bonus of being able to sell your source code or deploy local cloud if an enterprise are willing to pay extra for it.