No full definition, though it could have to do with fighting, and concerns of elbows flying regularly. I also suspect it has to do with facial recognition for fans. Remember that our current commissioner for a short time in the dawn of his reign gave players the option to take off the helmet, hoping that stars would do it; alas, one enforcer did it and the experiment was ended. It is different playing with a visor versus full facial protection, as a long time ago in a galaxy far, far away, I was a left winger on the checking line and at 16, I could ditch the bubble and graduate to an Itech RBE 2 when playing Juniors. Upon my re-entry into hockey 12 years ago, I eventually rigged up (with the help of an EQM friend) a similar visor and lower bubble protection that afforded the wide-open vision of a visor with sufficient jaw protection. Losing teeth in juniors and up would’ve been a badge of honour; as an adult playing beer leagues, not so much
So to answer the question, it is likely traditionalists and money driving the rules.