One reason that comes up every so often is that after someone goes extra crazy and kills a bunch of people his ( usually his for better or worse ) net presence

One reason that comes up every so often is that after someone goes extra crazy and kills a bunch of people his ( usually his for better or worse ) net presence is carefully scrutinized for any aberration that could have indicated he could and would have done it and then it is used to ask 'why didn't you some gov official do something?' And politicians like to stay in office. Oddly, as bad as this practice is, it is likely one of the lower threats to privacy in US.