Why do atheists keep linking Christianity and crime?

They never cite any sources. I think it's common sense that practicing Christians do not go in much for armed robbery and narcotics trafficcing. I'm sure most criminals are quite without religion,whatever show they might put on for parole boards (or,in some instances,sincerely convert while incarcerated). I keep seeing these postings claiming "most criminals are Christians". Were did atheists get this strange notion?