I don't think that it's cruel just to use an animal for entertainment IF it's happy and healthy and its needs are actually being met. But I don't believe that orcas' needs
are being met in captivity. The kind of setup you see at SeaWorld seems to be working okay for dolphins, but dolphins have drastically different behavior in the wild from orcas.
Bottlenose dolphins live in "fission-fusion" groups in the wild. They aren't all related, and they come and go as they please, kinda like human society. But orcas are extremely social and close knit. Even the males stay with their mother's pod their entire life. Studies of orca vocalizations even show that distinct groups have their own "dialects"--different pods don't make the same noises, or the same combinations of noises. Even when they are captive bred there are still "social deficiencies". I remember that one of the details mentioned about one of the "aggressive" orcas was that they'd been keeping him totally solitary, no other orcas around at all, just the trainers when they decided to interact with him.
For an animal
that social, that's really, really sad. I don't know that keeping a parrot or even a dog in similar conditions would be considered humane.
The effect on their physical health alone should be an obvious indication that something is very wrong with how people are handling their care. The norm is for captive animals to live
longer lives than their wild counterparts--this is true for birds, wolves, cats, ungulates, reptiles, etc.--but from what I've read, captivity apparently shaves decades off of an orca's life. If an animal that can live to be 60-70 years old in the wild, is only surviving 20 on average... what is going on?