Strange phrasing, but as Thailand does not have English as a first language, I'd say it's a bizzare terms for a safari park, and perhaps they use it to separate themselves from other Thai zoos, some of which have rather dubious reputation.
'open' refers to the large open-plan aspect of the zoo (although many of the cages - in particular, those for the cats - are no bigger than you find in city zoos, simply spread over a larger space)
