Because you may be punished if you are caught, answered the athiest teacher.
And if I am sure that I wont be caught?
Then you dont steal because you will hurt someone else.
And if I dont feel bad about that?
How do you suppose this colloquy went with little Jim Bakker? Or Kent Hovind? Or Benny Hinn? Or Richard Roberts? Or perhaps these miscreants?
My point in the little dialog with the atheist Sunday school teacher was that there is no premise that is consonant with human experience and human nature that will allow a rational argument for doing right in all cases when self-sacrifice is sometimes required.