This is really a question about cat ownership in America. Or a comparison between the attitudes of cat owners in North America and those in Europe and elsewhere.
In the United States, cats have traditionally been allowed to roam free at their will but in the last decade this practice has changed. More and more cats are now confined indoors. This cannot be said about the UK or other European countries.
There are perhaps three factors as to why a relatively high percentage of Americans keep their cats indoors at all times. The first is that there are more predators in America. They consider the domestic cat as prey. There are also many declawed cats in America and veterinarians generally advise owners to keep these cats indoors because they cannot defend themselves. Another reason is to protect the environment. There is a greater awareness by people in general with respect to environmental issues; one concern is domestic cats preying on native species. Cat owners are aware of this and increasingly so. You might wish to add a fourth reason which is that Americans more commonly see the home is a more natural environment in which the domestic cat should live.
In a preliminary study of 256 households surveyed from 1993 to 2003, the scientist Bernstein found that 50% of 503 cats were kept indoors at all times. Of those allowed outdoors only one third had unrestricted access. This survey took place some time ago.
An article on the vetstreet.com website states that 75% of their readers keep their cats indoors. The same goes for veterinary professionals they say. They also say that 58% of veterinary professionals and 51% of readers of their website believe that their cat has always been an indoor cat even before they adopted the cat.
These figures are higher than the earlier figures which is unsurprising because there is a trend towards keeping cats indoors in North America. I had thought that around a third of Americans kept their cats indoors but that figure seems to be outdated.
In the UK I would be surprised if more than 10% of cat owners kept their cats indoors at all times. The overriding culture on this matter in the UK is to let cats roam freely. I believe this attitude should change in England in particular as the country becomes more crowded with more traffic. I believe that the attitude in England that cats should be allowed outside is outdated. But it is deeply ingrained as many cat owners believe that their cats need the freedom to express themselves and behave normally. And there are no predators in the UK interested in preying on domestic cats. This removes one obstacle to allowing cats outside. However, as is the case in America, there are cat haters who like to poison or shoot at cats and this is another common reason why cats should be kept indoors.
In countries other than those in North America or Europe, I would expect to find far more community cats and far less of an attitude that cats should be confined to the indoors. Indeed I would expect almost no domestic cats to be confined to the indoors in many of these countries such as in Central and South America, Africa or in Asia. It could be argued that this is as a result of a less refined or less well-developed relationship between human and domestic cat in some of these countries