And how is it any different in the typical evangelical Sunday School? There you shouldn’t steal because God says it is wrong because it hurts other people. Still those who don’t care about hurting others won’t be dissuaded, so Christian Sunday School is no different. Maybe you could threaten divine punishment, but you can’t threaten people with hell for stealing because either they don’t believe in God, are going to hell by default, and don’t care that you think they’re going to hell, or they believe in God, are once-saved-always-saved, and will go to heaven regardless of whether they steal or not. Maybe you could tell them, “If you steal, God will kill your dog.” Then they might avoid stealing because they are afraid of getting caught, once again making Christian Sunday school no different than your example.
As you note . . . “because God says it is wrong” IS what is different. Without God there is no rational basis for altruism.
I have a thought here that maybe can be illustrated by an analogy: see if this makes sense.
I suppose normal people have a natural drive to use all of our organs and limbs and capacities. There could hardly be a healthy child who does not use his arms because he "forgot to" or "it didn't occur to him," and the same goes for your legs, eyes, ears, teeth, genitals, brain, whatever. We have a drive to use our capacities.
Now, people have the capacity to transcend themselves: to know, love, and attach themselves to something bigger than themselves. They will direct their daily choices, strive, and even die for this "bigger" thing. There are people who get all wrapped up in the U.S. Marine Corps and those who get all into political liberalism, or conservatism, or scientology or college footbal or communism or whatever.
Now, the biggest and most satisfying attachment would be to the biggest and most comprehensive entity: to discernthat you were acting in concert with the meaning of the Universe and the Maker of All.
That's what the Evangelical Sunday School or any religious education and culture should deliver: the sense that the One who made the "Law of Thou Shalt Not Steal" is the same One who made the "Law of Gravity." The One who said "Honor The Father and They Mother" is the One who calibrated Electromagnetism and triggered the Formation of the Universe.
In other words, the Moral Law is in harmony with, somehow, the structure of Reality itself. And in harmony with the ultimate reality, the One who's self-definition is "I Who Am."
If you have a sense that obeying "Thou shalt not steal" puts you in synch with God's will, and disobeying it puts you out of whack... Then you've got a mighty reason to learn about, love, and live in accord with Divine Law. It satisfies your capacity to transcend your little self and attach to the Great and Good, which is what you've been your whole life longing for.