I agree with you about birds I do not think it is fair to enclose them, fish however are a different matter. Some fish adapt very well to aquarium life and in the wild will only have a small territory regardless of the size of the water they are in. This makes them suitable to live in an aquarium. Fish will not tolerate stress whatsoever so they would not survive in an environment that was not suitable.
There is one benefit to the aquarium hobby, it has help species that have been depleted in their natural habitat be returned by aquarium bred fish, how can that be bad?
Others such as Mbu's are treated terribly in Africa as they are a food source, in the dry seasons they can end up living in small pools and slowly drown or overheat let alone end up speared or in a net. They are being overfished and will eventually dye out in their natural habitat.
As a parent I think its really good for kids to have animals around them, it teaches them responsibility and duty and care, that said my other hobby is my allotment where I can teach them how to grow their own fruit and veg as well.