Think of the hard cover books she wouldn't have sold if people could load the books onto their e-readers for significantly less royalties to her.
I may be wrong, but I suspect that this statement is false. Just because an e-book is released doesn't mean it has to sell for mere pennies; many e-books sell for $14.99 upon release (this is becoming more and more popular). With the difference in production costs and delivery costs of e-books vs. hard covers, the amount of money made per copy would be almost the same. And remember, you can usually get the hard cover at B&N when it comes out for just a little over $20, not the cover price of $27.99 or $32.99.
Many people who use e-readers will not buy a hard cover, and many people who buy hard covers are not going to go buy an e-book. There is some overlap in the markets to be sure, but the idea of an e-book is not that it's cheaper, it's that it saves space on your bookshelf, when you travel, or when you just want to go to the local coffee shop and bring your ENTIRE library with you because you're undecided on what you want to read.
I suspect that Rowling, like many authors, is afraid of the digital age.
-J. Mark Byrnes