I am not so sure you can say that. They may not have known evil (i.e. they may not have actually committed sin), but they certainly knew there was at least one thing they could not do (i.e. eat the fruit of one tree). So they at least knew there were allowable actions, and forbidden actions. God specifically told them not to eat of the tree lest they surely die. This certainly shows they knew of the consequences of their actions, and implies an understanding of good and bad. Also, when Adam was naming the animals, and then he finally got to meet Eve, he stated that at last he had met flesh of his flesh, bone of his bones. So Adam at least knew that bestiality was wrong, and that only Eve was a correct mate for him. This shows that Adam at least had a rudimentary understanding of right and wrong. I think the tree is of The Knowledge of Good and Evil. Meaning that you know (i.e. have experienced, have acted out) both good and evil. It is through the choice of eating the fruit that they came to know evil, that is, experience evil through their own actions.