I always feel a bit uneasy about Zoo's too. On the one hand, it's an opportunity to see animals we'd unlikely see otherwise, but on the other, they are away from their natural habitats and I suspect that Zoos must continue to sell the idea that the animals are there for their own good, and that they're doing things to help the animals - as opposed to profiting from the fascination of kids, and the desire for parents make their kids smile.
That's it, you put the dilemma very well - I agree. And for us is difficult to judge which zoos do contribute to animal welfare in the wild and which don't...