That is right, Steph. Rosa Parks did not violate any state or national law, so far as I remember. It was a RULE of, as you say, tradition; and bus drivers saw that it was adhered to. People, white and black, would come from other areas of the country and, not knowing the local ways, whites would make the mistake of sitting in the back and blacks of sitting in or towards the front, and they would be corrected. As I remember, it took only a word from the bus company owners or managers to drop the rule. I am not trying to stand up for segregation here, but only to tell it the way I remember it. The same was true for the soda fountains and lunch counters, etc., where, in the South, if you were black you could only order from down at the end of the counter while standing up, and take out. Simply local custom. Once the manager said they would go ahead and serve blacks sitting down, it was done. These were old Southern ways, and, like a bunch of barnyard chickens whose territory was invaded by a chicken way down in the pecking order, white males got really agitated if anyone made a mistake regarding the various black & white protocols. They could be dreadfully nasty and demeaning, and I hated it. They really did not want to see things change, because, and this is the basic truth, they clung to the pleasurable sense of superiority they had, regardless of possibly being lacking in all other respects. This is also why you saw efforts to change things coming principally from the well educated and well heeled. The upper and middle classes did not need that special status, but instead came to feel strongly about the unfairness of the system.
We have the same thing, in a different form, but the same basic human instincts exhibiting themselves in all their ugliness with the systemic hatred of "the other." Gays, Women, Immigrants. "We have to be taught, before it's too late, to hate all the people our relatives hate. We have to be carefully taught."
And then there is the law. THE law. The very ancient law: "Love One Another!"
I remember reading about conscientious objectors in World War I going off to war as medics. They were perfectly willing to die in duty to their country of birth, but not willing to kill. I remember knowing some of these in World War II, where they were more accepted by their communities than was true in World War I. People were more understanding, and not so quick to call them cowards. I am now and always have been in sympathy, as there is no way under the heavens that I could kill another human being.