You are assuming that attendance at a "loud" music event must inevitably lead to hearing damage when this is obviously not the case.
By your logic here's a partial list of things that should not be allowed due to their volume level:
Rock Concerts
Gunfire
Jets Taking Off
Cheering At Sports Events
Chainsaws And Lawnmowers
Emergency Vehicle Sirens
Helicopters
Countless Pieces Of Industrial Machinery
Race Cars And Motorcycles
Fireworks
Trains
Ambient City Noise
et cetera ad nauseum
Then there are the other non-volume related hazards that should be restricted some of them potentially much more damaging than excessive volume.
Smoking Tobacco
Drinking Alcohol
Operating Motor Vehicles
Working With Toxic Materials
Sexual Intercourse
Working With Hazardous Machinery
Fall Hazard
Threat Of Violence
et cetera ad nauseum
Prohibiting the above would prohibit: fire fighting, policing, a great deal of manufacturing, farming, continuance of the species, participation in many sports, driving to work et cetera ad nauseum. All of these things can be avoided by choice, reasonably protected against, or are of such benefit to society that the risk is acceptable.
You do not have the right to restrict the actions of another based on the potential harm those actions might cause when all these criteria are met:
A person will only be subjected to those actions by choice.
When those actions can be fully protected against with commonly available safety equipment.
When there is arguable benefit to those actions.
....in other words when it's none of your shaging business.
And then there's the fact that if you play amplified music of any sort you're being an incredible hypocrite denouncing "hazardous" volume. Unless you're playing at less than 90 decibels (which if you're playing amplified rock or similar music you most likely aren't) then you are certainly presenting your audience a hazardous volume level.
In the words of the sages, "We've already established what you are my dear now we're just dickering over the price."
