They probably did siphon gas. Who knows how long they'd been driving after they took off? They don't have a trunk full of gas cans.
I don't know how you can blame him for luring the herd to the house. Was he supposed to run into the walker infested woods with Carl? They didn't have much time to consider options. The cars were at the house.
EDIT--if that was the spot where they originally lost Sophia, they'd already siphoned the gas from those cars. It's not like some walker is going to stagger along and fill them up.