The child should have been in the cart or she should have had him by the hand. A 3 yr old has no judgement as to what is safe and what isn't.
When you start blaming a parent for not doing something IMPOSSIBLE to do, I think folks get a little annoyed.
It is not impossible to take care of children in a store and make certain they are safe.
But a 3 year old, an OBEDIENT 3 year old, can be told to sit right here, honey, and play with your car. The mother could have still been well within sight of the child, and he of her, while she looked for what she needed and glanced at him from time to time to make sure he was still there and behaving.
I don't see a thing wrong with that. Now, IF we were to find out that the child was pulling on the mirror or banging it and the mother didn't notice, I would find the mother to be a problem.