You're making an assumption - don't forget that in today's world, many 3 year olds are too "big" to fit in the cart.
Of course, that would mean, with the attitude of many here, the mom deserved to lose her kid becasue she let him get fat!
I'm assuming that most parents are smart enough to control their kids. When mine got too big for the carts, they were either in the basket or holding the pocket of my jeans. The MINUTE they let go I knew it and they were back by my side where they belonged. As they got older, when Hubby was along, he'd say, "where do your hands belong?". My boys put their hands in their pockets and stayed beside us.
Was his hand too fat to hold?