Because the venue being an advantage is pretty much a myth and the line is basically being set by the way the money is falling. Plus many people look well beyond polls. Anyone who watched the first game knows that Bama outplayed LSU and probably should have won.
How's it a myth if a clear majority of screaming fans inside a dome are rooting for one team? You make a fair point about the first game (which I didn't watch) but if Alabama lost at home because they had to keep going to an unreliable kicker, I don't see how that makes them a favorite now. Betting action certainly moves the line, but I wonder how many points Saban gets over Miles.
I don't dislike strong defenses and the score doesn't show it thus far, but Alabama is playing well against a strong defense. LSU hasn't done much at all except bending, but not breaking.