There should be regulations in place to ensure the safety and health of the animals in the zoo's care. This shouldn't just be left for public zoos. It is important that the staff and owners of private zoos know what they are doing when it comes to caring and helping the animals that take up residence there.
The mistreatment of animals is a serious crime, and any place that houses and cares for a large number of animals should be licensed and inspected. Private zoos should be held to high standards of cleanliness and animal care to ensure that the animals they keep are not being mistreated.
It is my opinion that when zoos and other entities are regulated and inspected regularly to uphold these regulations, it prevents abuses towards animals, people and funds. When there is no inspection, there are no expectations being set for the zoo; the licensure sets the expectation of conduct, and the inspections ensure they're being carried out properly.
I believe that any zoo that is not licensed and inspected would likely have animal mistreatment in some way. They may have poor conditions, lack of management or other negative factors that are harmful to the animals. Zoos should be places where every effort is made for animals to thrive; they should never be places that allow the possibility of animals being mistreated.