I'm not so sure about it being a mistake from Firaxis' point of view. They are in the business of selling games. Steam is a vehicle for developers to sell games, limit piracy, provide automatic updates, and a social network for gamers. I've been using Steam for nearly 2 years now with other games (L4D 1&2, MW2) and have around 150 friends on there. Last night I got to play the game for about 2 hours. In those 2 hours I had 4 people ask me if the game was any good? What's it about? etc. etc. I later saw one of them had downloaded the demo and was playing it, and another had bought the full game. I'm just one person who played it for 2 hours during a time that a fraction of my friends were online.
This type of "viral advertising" is a smart move on Firaxis' part. For every person who (usually wrongly) blames Steam for any number of problems and doesn't get the game they get even more purchases who normally wouldn't consider Civilization.
Steam haters need to calm down, if you're having problems running the game your frustration is misdirected if it's toward Steam.