I don't fully understand the situation, but liberty park is actually an optimal place for them because it is in some sort of legal gray area that means it's neither quite public nor quite private, plus the owner is sympathetic. Using public land raises questions about to what extent and for how long they can monopolize said land for their own purposes, which is less of an issue where they are, because technically no one has the right to kick them out (an occupier friend explained exactly how that works, but I don't remember the details. I think that's the gist of it though).