Depends upon the child. Some of you touched on this with "impulse control" issues, etc. Age and maturity puts some context into it. Some children you have to be more of an authoritarian with a hard set of rules or a "code of conduct" without having to explain,..."It just is." until such time the child is mature enough to explain. One of the handful of key differences between raising boys vs. girls is that boys tend to be more socially, emotionally, and empathetically disconnected than the girls at any given age, so often times this means you can sit down with a little girl then explain and reason based upon "How would you feel?" Whereas, most little boy's eyes are going to glaze over, it's not going to sink in, and you just have to make rules of conduct, and guide them with more positive correction. I don't know how many times a parent of a little boy yells out in frustration, "What were you thinking!!!" Well, it's obvious, most little boys DO without thinking, and will continue to DO until someone catches them and corrects them.
The interesting thing about the brain, it doesn't think in "negatives". In other words, you are up on a tall ladder, you start to get nervous, and then someone says, "Don't look down!". So, what do you do,...you look down. If a parent says, "Don't touch dad's gun in the bedside table drawer!" As soon as the parent leaves, the child is going to be handling the gun. Alternatively, if you are positive and say, "Keep looking up! You're doing fine." then that idea of looking down wasn't planted in their brain. In other words, if you want to encourage positive behavior, you have to plant that positive behavior in their brain. The second you plant a negative behavior in their brain, they are most likely going to do it. So, don't do it. In business, we don't deal with "problems", we deal with "opportunities". It's the same sort of mindset.
So, in this case presented in the OP, the mother may be partially correct in not punishing the bad behavior, per se, but should be following up and guiding the child towards a positive behavior without mentioning the negative behavior. It's a bit of psychology.
You can also play the game of "I am so upset with what you did." It's a bit of acting, but with small children you can really "ham it up" with them. Most small children get really upset when they see their mom or dad almost crying and sad, so if you are "crying and sad" with what they did, you can get quite a profound reaction from them. In other words, don't yell, don't swat them on the behind,...that can backfire and make them more stubborn and defiant. However, if you can act like you are so sad and disappointed, their little eyes well up with their own shame and disappointment and they will be quite apologetic.