Jump Ball Administration Question

[JoeTheRef]

On many occassions during the jump ball administration to start the game, I've witnesed several referees, including some partners in games I'm in, tell the non-jumpers that they can't have a foot on the division line or straddle the division line. This is for both the non-jumpers lined up around the center circle and those that aren't. Do any of you tell players this or have you seen this before? If so, what rule are you or are these referees citing should somebody ask them why can't we stand there? Again, I'm talking about the division line. Just curious.