It's not just battery manufacturing, it's everything manufacturing. It's clear that Musk (therefore Tesla) is addicted to the rush of developing and releasing new products but are loathe to dedicate time towards the boring, iterative process of manufacturing optimization. To the point of farming out as much manufacturing as possible to third parties (Panasonic, China).

It's clear they want to be a design shop in the vein of an Apple, as opposed to an industrial manufacturer like GM/Ford/GE. I would not be surprised to see them offshore manufacturing more and more in the future (see their recent China deal).