Well, if they won't even allow you to go into that section of the store without purchasing something, that is their prerogative, just as it's your prerogative to not shop there in the future. We vote with our wallets.
As for signings, maybe my worldview is skewed since in the romance world, all our signings at conferences are to raise money for charity, so your purse/totebag/etc is checked at the door ahead of time and you're not allowed to enter with any books, but it just would never occur to me to bring a book to a bookstore. If I already own a particular book by that author and I want to meet her and get her autograph, then I purchase a different book.