I don't agree with the statement that those who behave morally are doing so exclusively because of some sort of fear for retribution, in this life or the next.
I'm an atheist, and I truly don't believe there is any judgement in the "next life" or any karmic underpinning to our actions beyond cause and effect. However, I don't go around murdering and raping and being a monster out of fear of prison.
I don't know "why" I'm not a monster, but if we were to speculate, no matter what you take off the table, the simple fact of the matter is that we would not have survived as a species if we did not evolve some sort of base moral code. So in absence of any other evidence, you can say with certainty that there is a very high likelihood that it's just a part of us as a successful human race. Most of us just don't go around killing, raping and drowning kittens because we don't want to, not because we want to but don't want to end up in a prison cell or a fiery pit.
To have that sort of outlook is hugely cynical and flies in the face of all the evidence we see on a daily basis that the majority of us do actually care about others... and not because we think we'll get a medal at the end of the race.
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