One of the arguments that we vegetarians, animal lovers, and human beings in general like to give is that we do not support killing, as long as it is avoidable. That we are pro-life in some sense of the word (NOT the abortion sense). Now, suppose there is a problem of stray dogs in a city. We would probably not support indiscriminate culling of stray dogs. We would, however, probably not be too opposed to catching hold of male dogs and performing vasectomies on them - soon the existing population would die out, and the problem would be solved. Elegant solution, eh? Not cruel at all. What we are disregarding here is that we are still culling potential future life. So then we are not really pro-life in the real sense. More like pro-present-life or more simply anti-death or anti-pain.
Incidentally, Islam says that it is pro-(human)-life and it goes all the way, including potential future life. Hence very conservative Muslims frown upon masturbation and condoms.
But coming back from the digression. The anti-pain policy explains why we oppose poultry farming. It is true that those chickens would not exist, or ever come to life, if not for the fondness that human beings have for the taste of their flesh. However, what good is life without physical comfort, no freedom - no meaning? Not existing at all is probably preferable.
But octopii and squids do not have a central nervous system. They can't really feel pain in the conventional sense of the term. Why oppose eating them? One argument could be that when fishermen go to catch them, they try to flee. Hence, clearly they do not like getting caught. Therefore, even if they can't feel pain the way human beings do, they do 'feel', if I may use the term, some sort of discomfort or resentment at getting caught. Discomfort is bad, hence catching octopii and eating them is bad.
But now suppose tomorrow scientist develop a new breed of chickens that do not have a central nervous system. Suddenly the pain argument goes for a toss. Also, consider that scientists manage to ensure that all these chickens are born brain-dead. Hence, they can't feel pain and can't even fail discomfort. Heck, they can't feel at all! They're born a vegetable, live as a vegetable, die as a vegetable, but still taste like chicken! I can hear a voice inside my head screaming, "This is wrong!" "This is immoral!" "This is unethical". But why? There is no pain. No discomfort even. The very purpose of existence of these chickens(if I may call them that) is to provide human beings with the soft, tender, succulent taste of chicken that they so love. "But its wrong!", the little voice still says. Why, though? I don't know. Do you?