I just never did that. It only takes a heartbeat to lose something precious. And why would she sit him in front of a mirror where people may walk? Or where he could be hit by a cart? I'm sorry but I can't agree that that is responsible.
Because YOU don't find it acceptable to let your child be 5 feet away and glance at them periodically, doesn't mean this mother was negligent. Heck, I grew up in California and heard about babies drowning in their parent's pools every summer. I wouldn't own a pool with a toddler around. But people do.
Some mirrors, especially those for trying on clothes are recessed. Unless someone needs to use the mirror, the child might be safer there out of the flow of traffic. Trying to navigate a cart around racks of clothes is a huge pain in the butt and it is very easy to loose a child who is supposed to stay next to you as you walk around the rack.
I have actually told my children to "sit right there" while I walked from rack to rack, glancing at them and watching for movement out of my periferal (totally misspelled, I'm sure). It seemed a better idea than trying to flip through a rack with one hand or freak out because he's gotten around the bend of the rack and I couldn't see him from where I was.