When I think about it, you're right. Their cages are like the equivalent of a dog cage but I've always thought of them as more of a fish tank. It is pretty cruel to keep them locked up as it would be for a dog, but I don't think birds have to constantly fly. Bird food is so light that people use it as an idiom to describe what may as well be an empty plate. I doubt it takes much to avoid health issues.
I wouldn't say 'cruel' ,birds are less adaptive compared to other pets like dogs or cats.The things is....such creatures won't feel comfortable ,no matter how much time they have spend in a cage (ofc there may be exceptions)