Adam & Eve were just using empirical research on the fruit. Given that God talked to them and they accepted it, here they find another thing that talks to them and they see nothing wrong with that.
The only way to find out the truth about the fruit was to eat it.
If God were a rational being, God would've said, "I'm sorry, but the talking snake lied to you. Seeing as how I gave you no preparation whatsoever in dealing with other spiritual beings, I'm going to let this one pass. Nothing will happen to you for eating the fruit. Just a warning, don't listen to any talking animals, no matter how convincing they are. And I will try to do a better job to reign in my creations from misleading you. To make sure that this doesn't have a chance of happening again, I'm going to put the tree on the highest mountain, so that you'd have to really go out of your way to eat from it, in which case you would really die. Now go back to procreating, please."