Amazon will never use SD cards in its devices because it fears content loss. If a user could dowload content, they could remove it. I've recommended that Amazon use internal memory for its content but allow the use of SD cards for system memory and personal space, but I'm not holding my breath.
Another peeve I have is that Amazon doesn't allow the installation of Ultraviolate software because it's competition to its own movie streaming service. But it sells DVDs that have that service included in the price. I keep hoping that someone will come up with an externam solution of some sort as an end run around the problem.