The first reason is that junior students often have modest incomes — imposing a fine may put unnecessary strain on their daily lives.
The second reason is that a fine alone only creates pain; it does nothing to help them actually correct the mistake.
A better approach is to have the junior student write a reflection on what went wrong.
What mistake was made? Why did the mistake happen? How can this mistake be avoided going forward?
The reflection must be reviewed seriously — never treated as a mere formality. The student must fully and carefully reconstruct the whole picture, reflect deeply, and think through a concrete plan for improvement. If the reflection falls short, send it back for revision. If it takes a hundred rounds, revise it a hundred times. Once the reflection passes review, share it with the entire department.
This approach is far more effective than a simple fine.
No one is a sage — everyone makes mistakes. But a person should not keep making the same mistake over and over again. To acknowledge an error and genuinely correct it is the highest virtue. In doing so, the individual also contributes hard-won lessons to the entire department. That is a true good.