The answer here is: the government can restrict the time, place, and manner of free association in a content-neutral way (ie, it can't regulate based on whether the party is for Tea Partiers, Klansmen, or dubstep fans) so long as the restrictions are narrowly tailored to a valid government interest.
The 50+ people that attend the event have done nothing illegal (other than possibly trespass depending on where the party is and how they get there, and anything illegal they may do while they are at the location in question)
The event organizer who may or may not have endangered all their lives in various ways from fire hazards to toxic chemical exposure to food poisoning. They have done something most would call wrong and are rightly required to submit to the rule of law for the good of the people they may cause harm to and obtain permits proving that they have met certain standards we set to avoid letting people take advantage of the public.
I'll add that I think this may be in may locations burdensome and irritating, but it is important. In the same vein as building construction standards and health inspections in restaurants are important. I do wish it was less 'red tape' but its a matter of balance. Would be nice if the guests could simply sign a waiver, but where do we draw lines? I haven't sufficient ambition to be either a lawyer or politician that I wish to try drawing these lines. I only believe that their existence is important & I know where I feel they should sit.
It's easy to be cynical, so I am!
http://www.discogs.com/Autechre-Anti-EP/release/157
I like the way they also printed some of the sticker notes on the actual labels. You can see this if you click on the teal sleeve image at the top left.
That sounds unconstitutional.