In the UK, you don't have to worry about your cat being picked off by the local coyotes, foxes (our foxes do eat cats, btw), fishers, great-horned owls, and other predators. As a ranger, I have also seen what cats can do to the local bird populations. In Australia, cats have done a number on the populations of many of the smaller marsupials. It's not cruel to keep them in, it's thinking of the broader environmental picture.