Why 'should' atheists be moral?

[Neo-Scholastic]

Well, in short, "should" implies that such and such an action entails good or desirable outcomes, depending on what it is you value. In other words, it's a question of "what does it mean to be good?" to which an answer is found in one's sense and education.

Right, I would add that while people naturally pursue what they perceive to be good, they "should" pursue what is actually good, as opposed to what is only apparently so.