I’d like to just interrupt a little and say that for a while, I actually held the attitude espoused by the OP, or rather the attitude it would espouse if it wasn’t being outright ridiculous. Obviously, no intelligent person goes around thinking that there are not significant secular and physical consequences to breaking laws and acting like an ass; so no intelligent person goes around acting like that, god or no god.
I achieved this base, selfish, purely consequence-based morality as a youth; my parents were both religious, and when I failed to buy into the silly sky-fairy, it left them with no avenue through which to teach or demonstrate non-religious morality. So, I went feral. This meant I was relatively nice, polite, and obedient. It also meant that I would break rules if I thought I could get away with it, or if the expected consequence wasn’t sufficiently threatening to dissuade me. The furthest I took this was stealing food, and eventually money from my parents. (Their consequences were far more lenient than the threat that civil law presents.)
I don’t do anything like that anymore, of course. What happened? I grew up, is what happened. I eventually noticed that if one games the rules and consequences, ever, it eventually comes back to bite you. And somewhat later on I noticed that if I held a consistent standard of behavior, I could define myself around it, and build pillars with which so support self-esteem that way, making it an even further desirable behavior.
As one who’s been there, I can say with definitive certainty that the crude selfish morality that the OP is painting as being the single inevitable choice for all atheists- -that’s a childish mentality. If a person thinks that that’s the only avenue available to atheists, it’s because the crutch of religion has prevented their own moral thinking from developing to an adult level.
If mswas actually did suddenly turn atheist, his moral capabilty might indeed be so stunted that he’d go through a period of acting like a spoiled selfish child too, but eventually, barring some sort of persistent mental block or getting shot first for his behavior, he’d grow into a proper adult and join the rest of atheist humanity.