When Amazon were created there were no .word TLDs. And as far as I remember, it wasn't even an idea that ICANN (or anyone) had discussed. So Amazon were perfectly happy with amazon.com.
But having a dot.word for your company makes quite a lot of sense when you've got so many sub-domains to play with. Things like film.amazon, cloud.amazon, music.amazon etc. So I can see why they went for it. Particularly as nobody objected at first.