The police will say that the low class youth are mallicious and deserve to be put in rehabilitation centers, while the upper class youth are assumed to be only practical jokers. Theoretically we know this to be true, but unfortunately we also know that we do not always practice it. Thankfully, we have the Torah to guide us on the right path in these social manners. Even though our legal system also forbids taking a defendants past into account, it is inevitable that discusion of the defendants records come up during the courts proceedings. Our rabbis totally understood this tendency in their above statement, therefore, they forwarned judges not to give in to this tendency.
Every society knows that lieing is wrong.
Our Torah has not only commanded us not to lie but has told us to keep far away form any falsehood, just like someone should keep away from the plague. We know that lies stick. When we hear anything, wether truth or falseshood we remember it. This is a function of the primary effect, which means that people have a tendency to remember what we first heard and to digest it and not to pay heed to what we have subsequently learned about the same subject, this has been proved time and again. G-d, who created man, knows that man is imperfect, he has told him how to keep away from falsehoods, so that he will not be affected by it.