Responsible pet ownership

There have been many posts recently about people's cats. I wonder how many realise just how much harm their pets cause.
I work with endangered wildlife in NZ which includes many different birds and lizards, even some almost extinct carnivorous snails and slugs which actively hunt worms.
I am constantly having to control roaming cats which people allow to wander freely at night to decimate the wildlife.
By control I mean trapping, poisoning or shooting. I take no pleasure in killing peoples pets but it is necessary to protect native species from extinction.
Please folks, if you're going to keep predators such as cats for pets, at least keep them caged, after all people keep birds in aviaries, goldfish in tanks, why can't you build a big cage for your cat?
Recently I shot a black cat, obviously well fed and cared for. Inside its stomach were 29 dead lizards of different species, 2 of which were thought to be extinct.