I knew what the complaint was going to be as soon as I started reading the letter. The one thing everyone agreed on in the letter to Target is that stores can't post signs for everything, and I guess this is just one of those things that doesn't require a sign, similar to a sign stating no men are allowed in the women's dressing room.
Children should not be left alone in a store. Being a paying customer has nothing to do with it. Your child may be mature enough to be on his own and not disturb others or damage merchandise, but since B&N can't evaluate the children who enter the store, it's easier to just have a policy. It always made sense to me never to leave my children alone anywhere. They could disappear in the blink of an eye. Just curious though... why were you embarrassed?