So tell me, then - how far removed do you consider these creatures from the wild? Because I agree with your point, if we're talking about something assumed to be on the same level as...I don't know, a dog, perhaps, or some other domesticated creature. As it stands, however, these are animals that were captured directly from their natural environments, and can be released directly back into said environment with what's assumed to be no diminished capacity for survival.

To clarify a bit, there would be different expectations placed on a person adopting a dog, and a person looking to adopt a tiger - the latter is generally expected to respect the natural instincts of the tiger and not encroach too far on its own inclinations, while the former is expected to discipline the dog. To do otherwise in both cases would be considered abusive by most, after all.