Most games require online already. I don't know why most people were freaking out about it rather than just being a little disappointed. I saw the argument that they can't play in a subway, apparently they have their signals blocked off since a lot of the time, they're underground from what I've seen which is understandable.
(I've never been on one, as I've lived out in the country my whole life and the capital city in my state, Columbus OH in my case, I believe doesn't have subways since we use buses. Don't know about Cleveland or Cincinnati.)
I do go camping once or twice a year where there's no signal so with my experience with Pokemon GO, I can agree it can be a nuisance, but I don't think it's anything that can vastly hinder the quality of the game. It's just a norm that absolutely most people are used to. As long as it doesn't use a crap ton of data during long sessions, I don't think it'll be a big deal.