You cannot just disconnect from the internet with Steam. You have to switch it too offline mode first.
(Top left of Steam window, click steam, click go offline)
Once offline, it'll remain that way and you can play to your hearts content.
As for use of Steam, it's been known for over half the year that it would require Steam for online authentication (Surely you saw mention of Steamworks) if you read nothing of the sort, then the information you had was incomplete.