I wondered about that too! I think the difference is that cooked rice is just an absolute glutinous mass of mush when it reaches your intestines, and parasites love that. Raw rice, on the other hand, is probably still pretty darn hard, and maybe acts as a broom to sweep things off of the intestinal walls, and probably also has some of its absorbant properties intact, to suck in toxins from the critters. Just a guess, though ;)
Brown rice is probably better then white for this purpose, since it's harder and takes longer to cook.