I know this is an older thread, so I hope it is OK to reply to it with my thoughts on these matters.
I have no problem with keeping any pet or farming any animal for food. Humans are at the top of the food chain, we evolved to eat meat...we just do it more efficiently now. As long as the animals are treated fairly which most often is not the case, I have no issue with it. Some pets are the result of a mutually beneficially relationship thousands of years old, and we would still have those pets were humans in the wild, as the case used to be with wolves.
I'm unsure about the comments being skeptical of the intelligence of octopuses...certainly its possible that like with what happens with cats there are some instinctual behaviors being mistaken for intelligence....but there is too much evidence of actual problem solving, learning and reasoning. If I recall correctly it is considered one of the few animals that may displays evidence of meta-cognition.
Not to mention the numerous anecdotes of them displaying what could foreseeable only be intelligence. They certainly seem to compete or outclass many vertebrates long considered intelligent.
As for keeping them in a tank..same with any other pet, I see no reason it shouldn't be kept for entertainment and learning purposes....I want to make sure my tank is a nice place for it, but I have no problem keeping it in captivity, where it (assuming I do everything right) it would very possibly live a healthier, happier life. Watching people eat an octopus tends to make me uncomfortable though, especially when it's live.