This is C venues. They've been doing this for years. Festival “volunteers” are subjected to horrific working and living conditions and are boxed in by the fact that most of them are in company accomodation possibly without the means to escape home (generally London-area) until a pre-booked return ticket at the end of the season.
https://www.edinburghlive.co.uk/news/edinburgh-news/edinburgh-fringe-venue-firm-accused-23130671