I don't think there are any legal reasons. I think its because Sonic is not a dine in restaurant. You either have to sit out on the patio, go through the drive thru, or eat at a stall. I think the sonic franchise thinks that if the have them up north they might not get enough business because of the winter months. They think people will not enjoy sitting out in the cold temps waiting for their food when they could be in a dine in fast food restaurant where they can be warm and comfortable.