A Bad Dad

I was doing some grocery shopping at Shaw's yesterday. As I was waiting in line at the register I noticed the dad in line ahead of me shoplifted a big jar of Planter's peanuts. There was a toddler in the toddler sized plastic car thing attached to the front of his shopping cart. There was also an infant in a car seat perched in the wire seat where little kids used to sit before they came up with the shopping carts that look like big playground toys. The dad had a basket full of family food and the jar of peanuts wedged into the right side of the wire seat, on the side of the car seat furthest from the cashier. The man chit chatted with the young cashier as she rang up his merchandise. He had his arm draped over the top of the car seat and his body positioned sideways, facing the cashier and blocking any view she might have had of the peanuts. From my perspective standing behind him, the jar with its bright yellow cap was very easy to see. I thought about chiming in with an "Oh sir! You forgot an item in your cart." But I didn't. When I got out into the parking lot I saw their car parked across the row. The kids were in the car and the dad was putting the groceries into the trunk. I noticed the jar of peanuts was nowhere in sight.
Why would someone go to the supermarket, spend over $100 on groceries and then shoplift a $3 jar of peanuts? Especially with two small children in tow. I don't get it.