Until last year, in many of the states in the USA, there was little to stop people from keeping big cats as pets.
One may ask, what is wrong with that?
There is a number of problems with this, not least the fact that it is often dangerous as these lions tigers leopards or jaguars can escape and cause havoc for not only the owner but also their neighbours.
Added to this is the simple fact, that often the demand for these animals is not self sustaining, leading to further animals taken from already shrunk wild populations. It is a relatively well known fact, that there are more tigers in back yards in the USA, than there are in all the tiger range countries.
While some specifically breed specific subspecies, most owners are not careful about bloodlines, ending up with many of the (for instance) tigers being a mix of many sub-species, which means that they can never be reintroduced into the wild, so have no conservation benefit.