Here is why. Any defender of this argument must say why "good" and "bad" are objective. If you are going to say "good" and "bad" are objective because of God's fiat, then you must realize that in the end there is no difference between right and wrong for God, because God's character alone determines he will always behave well, and this contradicts the Bible. If you say that "good" and "bad" exist independently as objective categories apart from God, then you must admit that what is "good" and "bad" are features of the world independent of God's commands, and that makes the argument nugatory.
Objective morality being founded in God according to his eternal unchanging nature means that stealing isn't wrong because God says so, or because God apprehends so, but rather that stealing is objectively wrong because God, the omnicient, omnipotent, eternal author of creation is not a thief.