Why are teachers blunt and harsh wuth students and make them cry?

Man teachers can be harsh. One time tbe teacher I work with had a hard talk with a student and saying a 9 year old should not be jumping up and down and get out of their seat and now she has to wear a harness. She said, "Well since you wanna be a baby, you're getting treated like one. You're friends are shocked that you would act like a fool and push a student and that student could have been injured very bad. Now shw has too wear a harness like a baby. It's HER fault." She put her head down and started crying. The teacher got in her face and said, "Go ahead cry. I don't care. YOU chose to act like that and you have to wear a harness. That's YOUR fault! That's the truth. Pathetic."
I comforted the girl but the assistant said, "Don't coddle her. She's a big girl and she needs to see her own faults. If you don't give these kids tough love and let them learn to take responsibility for their own faults, they're not going to learn. They'll grow up thinking things are never their fault and blame others. Sometimes you need to give them a hard talk. She SHOULD feel bad." The teacher agreed and disliked when I was comforting her and I am too soft.
True but she didnt have to be so blunt and get in her face.