I've never been to a B&N signing but at Virgin I had brought a CD I'd purchased because I wanted it signed behind where the CD goes. I thought it would be easier to have it open before I got there rather than struggling with the plastic while going through line.
Anyway, I had to buy an additional CD in order to walk through the line. I'd imagine that B&N is the same. They have these events to generate money for the store. It doesn't do them any good if you bring your own CD and don't spend money there.
"All our dreams can come true -- if we have the courage to pursue them." -- Walt Disney
We must have different Gods. My God said "do to others what you would have them do to you". Your God seems to have said "My Way or the Highway".