The politicians (not bureaucrats: bureaucrats don't make policy) aren't making these laws up all by themselves: they're responding to the whims of the voters.

Jim Crow laws were "fair and just" at the time, according to most people of the time; that's why the laws existed. The people wanted them (well, the majority who had political power anyway, obviously not the Black people). "Fair and just" is simply an opinion, and according to the people at that time, those shitty laws were fine. They weren't getting their morality from lawmakers; the people at the time simply had bad morals.