" the default assumption has to be that they're greedy scum lining their pockets at the expense of young and skint musicians desperate to do anything that might lead to a paid gig someday."

Jack, this certainly isn't true of most Scottish festivals whereby the majority are run by volunteers. Any surplus gets held in reserve for the following year but this is rare as most depend on sponsorship even to break even. Yes, there are some big commercial companies profiting out of running such events but most "folkie" festivals are small affairs run by enthusiastic volunteers.