From my understanding the reason the rule was put in place is wrestlers were covering the holes in the sides of their headgear with tape. Hard head slaps caused several ear drums to rupture because air inside the headgear was unable to escape through the holes, and the head gear manufacturers were sued- but won because the headgear was not being worn as they had intended it. The NFHS needed to cover their rears from litigation, so they put the rule in place.