Kirk? Spock? Metaphor?
Atheists do not believe in God, because there is no sound evidence for God and atheists do not believe in things without evidence. Scientists tend not to believe in things without evidence, God is a thing one could believe in, so absent evidence many scientists are atheists. But so are plenty of non-scientists. This point is simple, and is utterly disconnected from morality. Santa Claus might illustrate some sort of imperfect morality as metaphor (not that I think that this is the case) but that that doesn't mean Santa exists, or that the argument for Santa depends in some way on whether or not the Tooth Fairy is needed to fill in moral gaps in pure Santaism.
Many atheists, like many theists, have an admirable personal moral system. Indeed, since they act in morally good way without any hope or expectation of postmortem supernatural reward or punishment, one could argue that a good atheist is a much better person than a good theist whose good acts are in any part motivated by hope of reward or to avoid punishment. Atheists tend to recognize that if heaven or hell exist, they exist right here, right now, on Earth and human action is the only thing that can increase the prevalence of the one and decrease the prevalence of the other. An observation that was reportedly made several thousand years ago by the non-supernatural empirical social philosopher atheist, Siddhartha, a.k.a. Buddha.
Atheists often try to live a life that minimizes both their suffering and the suffering of those around them, and to work for a better world for all because that's the only safe and secure way to maximize a better world for themselves and those they care about. A rational morality is actually quite possible without an imaginary source of supposedly perfect magical justice in a world that is quite obviously lacking any such thing.