I'll interepret this as a question about whether fish is meat, and then move on to the more pertinant question: Why does someone eat a diet of vegetables and fish without other kinds of meat?
Whether fish is meat is a purely semantic question. So is the question of why someone might eat fish and call themselves "vegetarian." There are many different possible diets, or dietary laws, and there simply aren't enough words to cover all of them. There's more than one kind of kosher, there's more than one kind of Ital, there's more than one kind of "vegetarian." If someone successfully coined a simple one word description meaning "I eat plants, seafood, eggs, dairy products, but not the flesh of land animals," then maybe people would use that. But then you'd probably ask "why do some xyzarians think that snails don't count as snails?"
Now, is it legitimate to decide I want to eat vegetables, seafood, eggs and dairy, but not other animal flesh? I'd say it's at least as legitimate as being vegan, vegetarian, Ital, or any other dietary definition you can think of. Meat eaters would just as easily, and fairly ask "why are there vegetarians in the first place? What's wrong with meat?"
As for me, I believe that the factory farming of land animals is cruel, and I also believe that meat is no longer a good use of land resources. When the population of humans was smaller, when our agricultural resources were limited, hunting and herding made more sense. But if you have to raise tons upon tons of corn just to feed one pig, using huge amounts of land resources, polluting the environment, and subjecting massive herds of pigs to terrible conditions, when the corn would better feed humanity, I just don't see how that makes sense. I also don't see the same problems relating to fishing. But that's just me.