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 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.
Bias 1: It's not reasonable to be generally suspicious of groups of people. Why would you assume they have no moral code? Just because they don't follow the exact rules you do doesn't mean they have no morals.
Bias 2: People are generally moral beings (for trustworthiness and "do unto others" values of morality). It is much more likely that if you think that people are
not generally moral beings unless they belong to your religion, this is more to do with how you see the world rather than how the world is.
Edit:
Another cognitive bias: I see plenty of immorality from religious people, and very little "love thy neighbour". I realise that the "Die poofter!" or "Die Muslim!" or "Die Jew!" contingent are only a tiny minority of religious people, but I'll bet good money that their fellow believers are able to overlook this obvious lack of moral code because they are from the same church.