Where do children get the idea that "Susie did it first" is an acceptable justification for an action?

I understand that children are not masters of moral reasoning, but this thinking seems, at least, to be an attempt at moral reasoning. From an adult perspective, it is hard to determine what the underlying principle might be. Where might children get the idea that other people acting badly is an excuse for them... show more I understand that children are not masters of moral reasoning, but this thinking seems, at least, to be an attempt at moral reasoning. From an adult perspective, it is hard to determine what the underlying principle might be. Where might children get the idea that other people acting badly is an excuse for them to do so, as well?
Update: Serenity, that's a good point, but for argument's sake, I am assuming that Susie is not my child. Sally is my child. I cannot punish Susie.

Thanks for all the good answers so far.
4 answers 4