I think we have to accept that not every citizen is a paragon of virtue. Some may have been in trouble with the police before, or they may not particularly like or trust the police. There are lots of reasons why people remain anonymous hence why Crimestoppers exists. Also, not everyone is brave or strong of mind or morals. That is life. I'm not saying these sightings are genuine but we can't just transpose our own values onto the general public and question why they don't behave as we would (or how we would like to think we would behave).