Whether something is a pet is a personal choice.
Animal cruelty laws don't simply apply to pets, they apply to livestock and many other domestic or wild animals under human care.
Animal cruelty laws are largely dictated either by whoever is in office at the time or the general public's views on such matters (the former should be dictated by the latter but often isn't which is why I make a distinction). If the general public thinks something is unfair there will be an outcry and people will move foward to try and change the current laws.
Our culture has much to do with views on animal cruelty. Culturally we have certain opinions on certain animals, and those opinions may also be dictated by the situation itself, not only the species.
Whereas someone clubbing their pet rabbit over the head because they don't want it anymore is deemed cruel, someone slaughtering a food rabbit is usually deemed more acceptable because these rabbits aren't necessarily socialized and are bred for nothing more than meat. Most people don't find it acceptable to simply torture an animal because it will be food, but they don't have the same emotional attachment to such an animal.
As far as things like people feeding fish to pirahnas, think about it this way. The general public doesn't know much about fishkeeping or fish at all. The general publics opinion is going to be based largely on the small amount of knowledge they do have. Most people think pirahnas are vicious killers and therefore MUST be fed live fish. Whether this fish is an arowana or not doesn't matter to most people. While views in the fishkeeping community may be mixed as to whether it's acceptable to feed them arowanas, feeders, large fish, small fish, mammals, etc., we are only a small percentage of the general population. Laws and ethics usually aren't dictated by a small percentage but by a culture in general.
What you should ask is what is the definition of pet? Most people consider any companion animal a pet. Any animal that people commonly have an emotional attachment to is generally considered a pet. Not everyone has an emotional attachment to their fish, nor a majority necessarily, so whether a fish is a pet is pretty much up to your own personal opinion. Dogs and cats are so often kept by people looking for companionship from these animals that there's little to no debate on whether or not they are considered pets and deserve legal protection from certain acts. Fish, however, aren't necessarily kept by most for that reason, so I guess it's not important to most people whether they're afforded protection from cruelty or not. Then you also have to define cruelty...so it gets pretty sketchy from there. A lot of people think that fish don't feel like other animals, so it is not considered cruel to do certain things to them that would be considered horrendous when applied to other animals.