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 ...
The citizen gives up his "freedom" to do whatever he wants, and thus to break certain laws, in exchange for the government's promise to protect his/her safety. Thus a citizen stops at a red light - thus giving up his innate "freedom" to pass through it - in exchange for the government enforcing red light laws that will enable him to safely drive down the street when the light turns green.
This is a very 'modern' theory of obedience to rules and laws, based as it is upon the idea that the government and its people have essentially mutual interests. Within Rousseau's vision, the State clearly exists to serve the People. Some earlier theories of jurisprudence relied upon more authoritarian principles of law obedience that, despite their age, still have relevance today. John Austin, with his The Province of Jurisprudence Determined (1832) suggested the idea of "habitual obedience", that is obedience given to a sovereign (or perhaps elected government) based upon a fear of sanctions. Crudely, people obey the law because they fear the consequences if they do not.
On a superficial level Austin's ideas seem self-evident: people do avoid breaking the law because of the consequences if they do on an individual level. However, if a whole system of laws is based solely (or nearly completely) on "habitual obedience" then it is by nature fragile and ripe for change. A frightened people has no loyalty to the system of laws other than that which is forced upon them. If they get the chance to successfully rise up, or if another member of the ruling class sees a chance at taking over, then it is likely to occur. Habitual obedience occurs within totalitarian states and dictatorships, but these are often short-lived, while superficially