Positivists claim that the legal validity of a rule is a matter of that rule's derivability from some basic conventional criterion of legal validity accepted in the particular legal system in question . . . The mere fact that a rule is just or reasonable will not make it a law; nor does the injustice of a rule demonstrate that it is not a law."
Legal systems are not static, immutable structures but rather mutable, and ever-changing organisms. The fact that they are constantly changing means that some constancy as regards to the need to obey the "rules" of a legal system is needed. Otherwise there is chaos.
The apparent paradox within the idea that "the mere fact that a rule is just or reasonable will not make it a law" and also, "nor does the injustice of a rule demonstrate that it is not a law" is in fact satisfied by an understanding that laws are, by their nature, limited in scope. Laws cover a finite number of situations whereas the experience of life for the series of individual human beings that make up a society is infinite. Thus it might well be "just" and/or "reasonable" to have a rule that people should be "kind to one another", but this will not make such a law feasible. Conversely, a rule that says that people of different races cannot marry (as occurred in the US South) is not made invalid as a law just because it is unjust. The morality (or otherwise) of a rule is, within the positivist sense, irrelevant to whether or not it is a law. It is a law because it is held within a complex legal structure that defines it as "law".
If laws are essentially arbitrary in nature, why should citizens of ...