In accordance with the unspoken rules of Great Plain City, people conduct their business during the day, while ghosts roam at night.
This distinction between black and white is a rule carved into the very ethos of the martial world.
Such rules don't require explicit clarification. The Magus, or Sorcery arts world, thrives on unspoken agreements. If laws and rules were too stringent and openly laid out, the Sorcery would cease to exist and devolve into a society governed by law, losing its essence.
The Magus World flourishes on unwritten rules—rules everyone tacitly understands. These rules form an invisible line that, if crossed unknowingly by the inexperienced or naive, will result in severe consequences.
This is why newcomers to the Magus World often struggle. Lacking experience, they fail to grasp these unspoken rules, making their journey fraught with difficulties.
Aizen understood this.