So in the UK I would be viewed as someone who keeps her cats in prison. I live in a two bedroom condominium and with the exception of letting my 2 cats on the balcony, I keep my cats inside.
I'm so glad the U.S. supports keeping cats indoors because many people cannot afford houses with gardens and must live in pet friendly apartments and/ or condominiums and if all these people were banned from having cats we sure would have a lot more homeless kitties(if it isn't a high enough number without this restriction), many more healthy cats would be put to sleep, shelters would be even more overcrowded, and there would be more deaths to cats from automobile accidents and other dangers simply because many people wouldn't have the option of keeping cats inside because they would be judged as cruel.
If I lived out in the country, I may consider letting my cats go in and out as they please but I may still place some restrictions for their protection, such as supervision or some kind of fencing.
In the UK, can the majority of people afford houses with gardens and are there just not as many homeless cats in the UK as there are in the U.S. that shelters can be this picky? Sorry, I'm just imagining what it would be like if the tables were turned and these were the rules in the U.S. To me they seem unfair.
It is becoming more common to keep cats indoors though, which means that more indoor cats are ending up in shelters and can be rehomed to indoor homes. Attitudes towards indoor cats varies from shelter to shelter. Some will rehome to indoor homes, others will only rehome certain cats to indoor homes (e.g. those with medical conditions such as FIV). Not all will insist on a cat flap (I think the cat flap requirement is so that cats that go outside can always gain access to a warm and safe environment - home - even when the owner is out).