The fact that both Barnes & Noble and Amazon.com agreed to push customers to the Pottermore site shows the power of J.K. Rowling's books. Pottermore will be able to collect customer information and credit card details, something highly valued by e-commerce companies.
In addition, it's widely believed that Pottermore is taking a bite out of the share of revenue that Barnes & Noble and Amazon.com usually take. In other words, the sites are taking less per sale than they typically would. It's sort of like the coup that Apple scored when they managed to get the Beatles into iTunes. Rowling's books are the e-book version of The Beatles.
Russ Grandinetti, Vice President of Kindle Content said, “For years our customers have loved reading Harry Potter books in print, and have made them the best-selling print book series on Amazon.com. We’re excited that Harry Potter fans worldwide are now able to read J.K. Rowling’s fantastic books on their Kindles and free Kindle reading apps.”
He's talking about you, B&N and Amazon.com.