I don't think there's much for Amazon to gain from publishing these sorts of internal details. Amazon's services are used by developers who are looking to tightly optimize their usage. If Amazon were to publish detailed internal information, it's likely that folks would start optimizing applications based on internal details that have the potential to change over time.

Secondly, I think that a lot of companies publish these "tech blogs" as a way to boost recruiting (look at the cool stuff that we're doing, don't you want to join us?). Amazon, of course, doesn't have a recruiting problem. If you want to work on the largest-scale systems, it's already a top destination for you.