If your event has an accepted etiquette then it is upto the leaders to enforce that etiquette. If the etiquette is vague or not established with the frequenters then you have to put up with any shit that comes along.

e.g., if you are in a public bar that is open to anyone then instruments, loud talking etc are all to be expected. The loud talkers may well be offended or put off by the singing, especially if someone insists on singing a Robin Hood ballad.

There are people on the folk scene who think it's okay to walk into a quiet local bar where the locals are enjoying a quiet drink, and start singing sea shanties at the tops of their voices; but not many thankfully.