"Rather than interviewing incarcerated burglars, they set out to find active burglars in the community. They drew on a network of people who they believed were likely to know criminals. Interviewees would introduce them to burglars who in turn would introduce them to other burglars. This approach introduces a selection effect, of course, but avoids the obvious selection bias arising from only interviewing burglars in prison."
Given the research question and real world limitations, I concede that some form of selection effect is perhaps inevitable. I am not sure, however, which injects more bias, only that different flavors of bias arise.