Why do people at work like to snitch to the manager/supervisor/director about simple things?

Why do people do these things. Do they think they are gonna get an increase in their pay, more paid time off, extra benefits or a promotion? This annoys me so much. And the people that do the snitching are the people who need to look at their own work habits. For instance, I work with at a preschool and the people there are so darn petty. Last week there was a parent who came to pick up a child a little late. She called prior to let us know that she would be twenty minutes lat but ended up coming a little later. As soon as the parent left my coworker came in and said "Im telling the supervisor she comes in tomorrow". Mind you, we sometimes stay 30 or 40 minutes late sometimes. The parent is hardly never late. Another coworker told on another coworker because she took extra minutes during break. But the person who told takes hours during break EVERYDAY!!! It's mainly done by older people. Its like elementary school at some work places. I don't like this because and someone could lose their job over something so simple such as "she took 3 minutes extra during break." Why cant people mind their business or talk about it with the coworker or whoever their having a problem with? DO SUPERVISORS PRAISE THESE TYPES OF PEOPLE? Whats going on?