It is most likely a deterrent to the larger portion of white belts who would like to see just how good a blackbelt is in sparring.
I once witnessed a white belt rolling with a blackbelt (said whitebelt outweighed said blackbelt by 25kgs) fortuitously gain a toehold on the blackbelt during this light roll, and thus advanced to finish with incredible gusto. The blackbelt, noticeably taken aback by how much momentum said whitebelt had publicized, tried to roll and then tapped mid escape.
The blackbelt asked to roll again, and dominated said whitebelt for 20 minutes, frequently tapping him from positions like mount, by driving his hips own to the mat, or from sidecontrol, by leaning all the weight from the shoulder, to the jaw.