BTW: UP/DOWN arrows and Backspace is redundant. Just use PAGE UP/DOWN or CMD LEFT (chrome, mac osx) for back.
At the very least the search input should be focusable by clicking tab ONCE... not 15 times.
EDIT: The whole Tab Order just makes no sense at all:
1. Try Prime 2. Sign In 3. Try Prime 4. Wish List 5. Shopping Cart 6. Departments 7. Fire & Kindle 8-14. No visible focus 15. Search input
I bet this has NEVER been A/B tested.