I think most zoos used to be "cruel" places, because people mostly didn't know or care about the animal's welfare, only their entertainment value (as in the "chimpanzee's tea parties" they used to hold up until not so many years ago). And confining animals in tiny cages where they had nothing to do but pace up and down all day is obviously cruel. But these days most zoos, at least in this country, seem very much improved and genuinely concerned with the animals' welfare as well as (or more than) with making a profit.
There are breeding programmes in many zoos for rare and endangered species. A creature that is not content will usually not breed. We also need to remember that many creatures probably don't have the same feelings about being "kept prisoner" as we do.
It's likely that, in a really good zoo, animals are actually more content than in the wild. Comfortably housed, well fed, protected from predators, free of too much stress, mostly protected from disease and injury, or treated if it does occur, life could hardly be better for many animals. The keepers even try to give them stimulation and interestiong activities to stop them getting bored (You may have seen "Animal Park", about Longleat Safari Park. In some of the episodes the lions were given huge "cat toys", (which they demolished pretty quickly but obviously enjoyed while they lasted); or the monkeys were seen excitedly collecting their real fruit ice lollies in hot weather.
I saw a TV programme once where research was done into what is needed to keep a hen happy! They decided that as long as she had food, shelter, somewhere to make a nest, and an area to scratch around in she was content with surprisingly little room.
Zoo keepers are also learning more about individual differences in what a particular species needs and likes. for example, meerkats depend on each other and need to be in a "family" setting. Flamingoes apparently will not breed unless they are crowded together with plenty of companions. Many small birds, such as budgies, also need to be in a flock and have plenty of space to fly around; while some cats and birds of prey may prefer to laze about for hours on end, just watching the world go by, and find it stressful to be with many others of their kind. Ferrets love ferretting everywhere and need lots of interesting small spaces and tunnels to explore and lots of stimulation. Mice suffer from agrophobia and hate being out in the open with nowhere to hide from danger - they would find such "freedom" terrifying! But a pack of wolves are used to ranging for miles in the wild and would need a larger space with plenty of places to explore or make dens. Bats need a cosy cave - quite small - to chill and hang out together and a decent amount of space to fly around in. And so on...
So, I suppose, whether zoos are good or bad depends on the zoo.
Thank you for the interesting feedback. I agree- some zoos can be terrible places, but most in the U.K ( the ones I know of) provide a comfortable environment for the animals, with features especially made for them. I personally love going to London Zoo, but other naturalists may find them a bit depressing seeing the animals in captivity.
I find it more depressing to see a single budgie alone in a small cage, or a goldfish alone in a small bowl in someone's house. Budgies need other budgies and the ability to fly around together, find mates, set up home. Goldfish need other goldfish to swim with, room to get away somewhere secluded, hiding places from bigger fish. Dogs too, being pack animals (descended from wolves) need companions, a pack leader to follow, obey and look up to, a nice den and other comforts. House cats are more independant, but even a cat likes the security of knowing its owner (or maybe from the cat's point of view its slave!) is around when wanted. Food, a comforting lap, a game or two when in the mood, a friend to say hello to and have a good rub against, favourite hiding places and vantage points, a comfortable, ordered and predictable world... Purrrfect!
Some of the zoos you see in the films are really horrible, and that may give the wrong impression to people. But the Zoos I've been to are really cool. However, in other countries people may not take into consideration that animals are just like us-they need a clean, safe place to live and lots of yummy things to eat.