Dolphins have no more intelligence than do pigs. Why are pigs "food animals" and not dolphins? Certainly the people of Japan don't need YOU to be the arbiter of what they are permitted to eat, after millenia of doing so. If an animal tastes good, than it should be a "food animal".
Actually, according to their encephalization quotient, dolphins are about five times as smart as dogs. This makes them about five to ten times as smart as pigs, which have approximately the same relative brain mass as dogs. (These figures vary from species to species.) I should note that dolphins are, according to relative to brain mass, considerably smarter than chimpanzees. They are one of the only other creatures on this earth that are self-aware (aside from chimpanzees and humans, of course). Please don’t pretend like dolphins are unintelligent.
I do find it amusing that you bring up the idea of “if it tastes good, it should be a food animal.” Sure, and plenty of people find domestic dogs and cats to be tasty. Plenty of people, for that matter, find humans to be tasty. That doesn’t mean they should be eaten. Any living creature can be eaten. That doesn’t mean that they should. There is more value in a dolphin life than a bovine life; cows can be contained by a single wire about hip high. (Their stupidity knows next to no bounds.) Cows also happen to be extremely tasty and provide significant nourishment. Hell, it’s almost like they’re a tailor-made food animal! They’re easy to maintain, easy to breed, easy to feed, grow huge - the list goes on. They provide leather and milk. Contrast that with dolphins, that provide little relative benefit compared to the difficulty in capturing them. (This also reminds me of the eskimos that shoot whales with .50 caliber rifles because it’s a “cultural tradition.”)
Look, there are plenty of things in this world that people enjoy that I find wrong. I don’t subscribe to this or any other sort of moral relativism. Japan is also home to some of the world’s most disgusting and revolting pornography. That doesn’t make it OK.