Well, Amazon is not very interested in making its APIs compatible with other cloud providers. But the whole thing is unrelated to POSIX itself. It makes just as much sense when you replace POSIX with e.g. HDMI cables.
Hard to say what Amazon is or isn’t interested in. The success of S3 is such that significant downstream client libraries are used by people who pay for S3, enough to provide a measure of api stability, simply out of customer inertia.