Why are we to believe that the gods could receive nourishment from the tree?

In Ishmael, the gods are doing their work the best they know how to, killing and let live, causing growth and drout. This isn't good enough for earths inhabitance who say things like 'How can you be so cruel'. So the gods find the tree of knowledge, of who should live and who should die. Now the animals are happy, they know that their destiny is correct. The gods however start worrying about man. They figure that man will come looking for the tree, man will eventually find the tree, man will eat from the tree, then be convinced he has the knowledge of who should live and who should die. The gods express fear in this because they know that the tree will not nourish man, but he will be convinced it has anyway. So they forbid the tree from Adam. My question is; Why are the gods convinced they themselves have the knowledge? Wouldn't they think they had it even if they didn't too?