Moreover: it's easy to trust each other if all you're doing is scientific research, games and pranks. But one day we woke up and found half of the world relying on computing in everything, from business to healthcare to national security. With regular people and regular day-to-day stuff comes regular crime.
Put another way, it's easy to trust a user if you and they both know that they can be tracked down and face disciplinary action if their behavior was deemed unacceptable.