<human>: Can you provide a brief overview of the text?

An 11 year old, let's call her Anna, just unknowing admitted to 'stealing' books. 
I tutor a few children after school and during a math-session break I overheard her talking to another girl, Jen, about books they like. Jen says she wishes she could have bought 'Uglies' (I think that is the name of the book), but she only had enough money for one book; so, she bought a different one. Then Anna says, "well, you read about as fast as I do, just hurry-up and finish your book and take it back to the bookstore with your receipt.  As long as you haven't had it over a week, you can exchange the book for a different one or just get your money back. That's what I do; that's why every week I have a different book."
Jen then asked, "You can *do* that?"
Before Anna replied I said, "no, that is kind of like stealing, don't you think?"
Anna then said, "I don't think so, that's what they do at a library, and it's free." 
I then tried, briefly, to explain to them that a library was meant for borrowing and education while the store was a business that depends on money to keep it running."
The girls kind of shrugged it off and due to the time restraints we dove right back into the math-lessons. 
Now, although I think her idea was brilliant, I still believe it is wrong of her to do this and i think this is a perfect opportunity to discuss ethics a bit, but I just don't know how to bring this up without sounding like a preachy **adult**!
Now, looking back on the situation, I wish I would have used a completely different argument, too. Like how the authors, publishers, & distributers won't get their fair cut because the books are being returned. 
So, what should I do?
Am I over thinking this, is what she is doing stealing, should I bring this up during out next lesson, this Tuesday, or am I overreacting about this whole thing?
<bot>: Smart young girl finds a way to get free books by quickly reading and then returning books to bookstore before return policy expires; how do I explain this is wrong?