An atheist's principles usually have to be deduced at the source, by e.g. talking to the atheist. This isn't hard, but it does take time. It usually doesn't scale to other atheists.

A moderately devout Christian's principles are likely ones you already know in some low resolution through cultural osmosis. This is reason enough to suspect that, ceteris paribus, people will prefer to engage in voluntary trade with the Christian over the atheist. It is less because of the Christianity itself, than because trying to follow a known standard for good conduct reduces transaction costs.