What is the source of the controversy? The intelligence of these animals is often cited... but apes are pretty smart too, and there is a lot less fuss about chimpanzees and gorillas in zoos. They are big animals, for sure, but giraffes and rhinos are bigger than dolphins and belugas and are much less of a hot-button topic (to be fair, elephants are controversial, but less so than orcas). To me, the answer is simple: it's the habitat.
We have spent decades revolutionizing habitats for terrestrial mammals, but dolphin and whale tanks have largely remained the same - a concrete box with a glass wall. Compared to an ape exhibit with tangles of real and artificial trees, or a savannah yard for giraffes, or a seal pool, with its haul-out spots, cetacean enclosures seem bare. Just water for swimming in.
The other day, it hit me though... exactly what are they doing in the wild? Swimming all day. Generally in deep (read: featureless) water.
It may be that in some respects, whales and dolphins are given some of the most natural conditions possible. Who cares if the floor is concrete and unadorned? How much time to wild cetaceans in deep water spend on the sea floor? Some, mainly to grab objects or take a food item, but they don't sleep on it. Less clutter in the tank makes it easier to swim and maneuver at speed. Constant keeper feeding, training, and enrichment keep the day varied and stimulating. Conspecifics provide lots of opportunities for social enrichment.
I would like to remind you, also, that SeaWorld was all set to construct far bigger orca enclosures at their parks. These plans fizzled after SeaWorld decided to phase out orca breeding.
I also suspect the antipathy towards whales and dolphins is also probably colored by the tradition of training shows, dolphins jumping through hoops and such. After all, manatees are large aquatic mammals that live in tanks, but there care is less controversial. Again on the other hand, most manatees in zoos are rescued individuals.
Every zoo and aquarium professional would like to see their animals in the best and biggest (though those aren't always synonymous) enclosures. Sometimes I feel like we become so reactive to perceptions of how things are, however, that we lose sight of what is actually going on.