It also isn't hard to just keep them in a cart. They can still play in the basket if they just can't fit in the seat.
You, and your mother, are assuming that the child was physically out of her sight, instead of considering that she simply wasn't looking at the child every moment.
If she wasn't looking, there were moments when he was out of sight. It would've simply been easier & less tragic if she had just kept him by her through some measure. Even if my mother wasn't looking right at me, she had a hold of me.
If my child is standing next to me and my head is turned, I may very well HEAR a problem before seeing it.
If they're standing next to you, they're not five feet away & unreachable when it counts.
To be honest, I find putting a child in the basket irresponsible. I cannot push the cart and grab the child if he tries to stand. I'd much rather he walk.
And if we wrapped children in bubble wrap and put helmets on them they would never get hurt. I don't think your mother's level of vigilance is bad, I think it is unnecessary for many children, mine included.
Now, how safe is that?