This is probably a major reasons why
religious people distrust atheist religion can be looked at as a moral consensus. It is a system of rules which it's adherents (sometimes nominally) agree to live by. By declaring themselves atheists individuals choose to publicly reject that consensus which leads to suspicion. The bitcoin equivalent would be a miner publicly supporting a closed source hardfork.
That Scientific American article describes that as un unsupported cognitive bias. Just because people distrust atheists doesn't mean there's a reason for it.
And just because many religious people feel they need religious rules in order to act morally it doesn't mean that people who don't follow religious rules can't act morally. It's more likely that religious people
need religion in order to be moral actors, just as BADecker wrote.
The statement I am an atheist as opposed to the statement I am an atheist and (insert moral code here) means the following to a religious person.
1) I reject your moral and spiritual code and have replaced it with nothing
or
2) I reject your moral and spiritual code and have replaced it with something I do not wish to share
It is reasonable to be suspicious of the values and morals of someone operating with no moral code. It is also reasonable to be suspicious of someone who knows and can anticipate your moral code but refuses to disclose his own. I fail to see the cognative bias.