It totally amazes me that some are more conerned about a sign/ policy not being posted than permitting an 8yr old to be left on his own in a store.

There could be a sign posted but again, for the most part common sense dictates that you do not leave a child on his/her own in a store like this. And you can post all the signs you want...parents will still leave their child unattended and feel wounded when asked to not do so.
You can still let the child have the freedom without hoovering over them. You can sit on the sidelines and watch or sit and read with your child.

I did however agree that the employees conduct could have been a bit less abrasive. And this it seems is the only reason the complaint was written...the mother was embarrassed...so it seems she cannot see beyond that.
She could have asked the manager to please have the employee not conduct herself in that manner...that most likely would have settled it. Instead she needed to soothe her wounded ego through a larger media.