Autofocus breaks scrolling with the keyboard, especially spacebar. It can also mess with screenreaders and blind users.
At least the search should be focusable by pressing TAB once instead of 15 times, which is insane.
Some users will of cause use the search box, but the majority clicks their way to the products they want using the menus. Amazon may have seen something similar, so not doing the auto-focus might make more sense, I doubt very much that they haven't considered it.