I think it's more that people don't know that you can get ebooks as files from other stores to begin with. But then also you need ones that don't do DRM, which

I think it's more that people don't know that you can get ebooks as files from other stores to begin with. But then also you need ones that don't do DRM, which complicates things further. But, yes, Amazon is also deliberately vague regarding those emails. Most of their documentation presents it as a way to "keep your documents in your Kindle"; the fact that books are a subset of those documents is not exactly highlighted.