When I buy a book on the Samsung Galaxy Tab Nook and click on read, after purchasing it, it thinks I'm doing Read in Store, and then tells me my session is up. If I then go to my library to continue reading, it doesn't remember my place in the book. It's annoying. I can get around it by not using the Read button in the shop, but then, what's the point of having the read button there after I purchase the book?
I can't answer the question as to why this is done, but, perhaps as a workaround, if you turn off wifi after purchasing the book in the store, this won't happen.