That implies a very weird argument. Like, if there is no punishment in the afterlife, one can commit any crime as long he is not caught. And as atheists don’t believe in such a punishment, they feel free to commit any crime or atrocity.
Well, if you ask me, that doesn’t tell us anything about whether there really is such a punishment or a god imposing it, but it tells us alot about the people who are only behaving themselves because of the expected punishment.
I feel much more comfortable with people not being criminal because they realise some behaviours are plainly wrong for some very rational arguments rather than out of fear.