> What exactly would the alternative have been in 2006?

Well, WebDAV (Document Authoring and Versioning) had been around for 8 years when AWS decided they needed a custom API. And what service provider wasn't trying to lock you into a service by providing a custom API (especially pre-GPT) when one existed already? Assuming they made the choice for a business benefit doesn't require anything close to a conspiracy theory.

And it worked as a moat until other companies and open source projects started cloning the API. See also: Microsoft.