"Our natural bullshit detectors are muted online; we can't rely on facial expressions and other physical cues for sensing lies, and studies suggest that without those cues, we're prone to generously fill in the blanks." Pretty much sums up why you should never trust anyone on the internet.

They need the attention, and they know there is a good chance to get away with it. It's like telling people not to rob a bank, but leaving it open at night, with no security under the impression that "all" people are generally good. 99% will use their inner judgement and will not do it, but there is a % of people that are prone to do it.