The government is in line with the people 90% of the time. So "just" here means what the people want, or some majority of them anyways. Of course what the people want could be unjust to some others, like if there's a decision to invade a neighboring country, then those neighbors will probably find that unjust.
Discussion
If its a majority thing than why not remove people from the equation all together. At the end of the day government is by the people. And just say what government does is justice.
I understand your point tho.