Paul has not told Chani that he has seen that she will die. Presumably, he kept this foresight from her because he wanted to protect her from the pain of seeing a fate that one is powerless to alter. Paul detests the clarity of his own vision because it makes him feel chained to a life that he cannot control. Because he himself wishes for blindness, he gives Chani the gift of blindness, allowing her to enjoy the present, unaware of her fate.