The way this was explained to me by my companies H.R. Dept. was the fear the company has of being sued. Say you get a nasty divorce, seperation or some thing along them lines and out of spite your husband or wife drops the family plan, then you or your child get injured and goes to the hospital just to find out that you are no longer covered.

Let's face it, if you can sue because you eat fast food and it made you fat or for spilling hot coffee in your lap this is not that far of a stretch.