So firstly, the empiricist in me would dismiss such claims right off.
Second, I would question the idea that a reincarnated person can be considered the responsible for the karma of their previous lives, because in losing their memories, they lose their previous identity. Even if some "soul" is saved in reincarnation, memory is such a crucial part of identity that without it punishment cannot be just. It be punishing another person in place of the original.
EDIT: Third, although this view does not necessarily justify treating disabled people poorly, it does justify the idea that medicating birth defects would undermine the karmic punishment. Since I don't hold this view, I have a practical, non-theoretical, reason to object to anyone holding this view.
