Scorebook Question

Had this occur tonight and it makes no sense to me...

In my first year as a coach I found out quickly that the home team's scorebook is not allowed to leave the scorers table during the game. It is deemed the "official" book. Well in a game tonight, the official book did leave the table during halftime. The subject was brought up to the refs and upon looking in the rulebook, the ref and the book were correct.

Even though it says the book "shall" not leave the table, there is no penalty if it does.

My question and concern is: Why is there a rule that says it can't leave the table when if it does there is no penalty?