Awesome. Let those flea-bag, disease carrying killers on zoo property. Smart. One of the major threats to native wildlife whether feral or let outside by their human(s). Cats that are feral, non-chipped/tagged should be trapped and euthanized.
Many European zoos have house cats on their property, mainly to hunt the mice in the stables, but they are not as common as here or visible to the zoo-visitors...
There are feral cats throughout the city, so they'd be quickly replaced if only those in the zoo were euthanised. I think they gather here for the fish and rubbish, but they are very effective at controlling the number of rats at the zoo. I haven't seen any cats inside the enclosures in the last couple of years, but that wasn't the case in the year that I took this photo: http://www.zoochat.com/928/cats-116834/