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I think it has to do with precedent; US law follows what was previously decided in similar cases. IMNAL, but I would speculate that this concept originates from the various arguments regarding how to interpret the US constitution. For example, does one expand the meaning of the constitution by creating new law, or does one simply follow the succession of law that has unfolded in the time frame since the constitution? The latter seems to prevail today.
This means that in America, you can do what ever you want as long as there is not a law prohibiting it. In other words, reason has nothing to do with it. If there is not law against it, you can do it. One egregious example is that you can make an ad of complete lies, and say, in the hidden, fine print that it is all lies, and you will have not broken the law. Just read the ads for losing weight. Now, what kind of person would do this? Well, one which wants to exploit others. This includes a number of people and businesses and virtually all politicians (well, most, anyway). But not most Americans.
Here is my point. Most Americans are very good people, but the government and politicians (and some businesses) are morally corrupt abetted by a legal system that has a basis in precedent rather than morality (or reason and sense if you object to the word morality). This must be very hard for non-Americans to understand; probably, most Americans have trouble with understanding this. But, awareness in growing.