The amount of content on the site has long outgrown its tab structure, but until now, Amazon has found creative ways to retain the tab structure while growing their store. I know Amazon does extensive usability testing so this new design is a vote of confidence towards the usability of flyout navigation and primary navigation on the left.
You may not see the new design when you visit the site. The FAQs on the new design explain:
Why do I see the new design on my home computer but not at work?
We’re still in our testing phase, and you may not see the new design all the time.