Throwing rice can be a hope that the couple have lots of children...definitely not a JW policy and considered "pagan." I have been to non-JW weddings who make the same request. It seems that rice does not agree with the digestive systems of little birds and it makes a mess
I went to a "worldly" wedding a couple months ago, instead of rice they gave everyone bubbles, which probably turned out better in photos!
Maybe the WBTS should change this policy, since the only ones coming into the "truth" are children raised in it!