I agree that goldfish are obviously not very smart animals and while they obviously shouldn't be treated with cruelty, they don't need an excessively large tank with amazing quality water. But this could then carry on to other kinds of fish, which do have special needs. Unlike goldfish, some fish require particular water conditions (ie. the Discus, which needs exceptional water quality and quite a low PH, as well as a well-planted tank). As was mentioned above, cichlids feel uncomfortable with a non-sand substrate.
And while goldfish are stupid animals, some fish are obviously quite smart. Oscars can be trained to hand-feed, and I've even seen instances where they were taught to swim through hoops and other obstacles in an experiment designed to test their intelligence. They can recognize their owners and quite often beg for food (my clown loaches often beg me for food when I walk into the room, but when my dad or sister walks through they usually become scared and swim away). If someone keeps 3 or 4 large, aggresive South American cichlids in a 4ft tank, should that not count as animal cruelty? They will be constantly fighting since their is nowhere near enough room to set up a territory, and the smaller/weaker one/s will eventually be killed.
Anyway, I guess the point is while this isn't a big deal for goldfish, there are a lot of other fish in the hobby which are not as stupid/easy to care for as the goldfish. Goldfish/guppies etc were probably targeted because they are largely impulse buys from unexperienced fishkeepers.