I have no idea, but I imagine that part of public availability relates to whether or not a model came up through stripping -- "dancing" is the nice word in America -- or is used to only doing studio work. There are certainly many public events, even outdoors, where dancers appear for their fans. The Boob Cruises were logical extensions of that, and of course pretty safe; the fans were registered passengers. I imagine that rules were carefully laid out, too.