If the safety protocols aren’t the issue, then what are you proposing is the purpose of this restriction? They are volunteers, so this isn’t a cost saving measure. That’s why I’m guessing it’s safety. I have great respect for hospital volunteers. I just don’t know how rigorous the protocols are. Maybe someone who works for a hospital could chime in on that.
The best way to avoid overwhelming healthcare workers is to wear a mask, wash your hands, and social distance.