Why should a child not be punished for accidents during potty training process?

Because a child will refuse to be cooperative if you punish them and they will begin to be afraid.

Learning something new have to be positive or it will turn into a trauma.You can't punish them for an accident and this is not something they are in control of yet. You don't put a 5yo to drive a car and then punish him when he crash. And you don't want a first-grader in diapers who's in therapy trying to get over his potty-training.