As the author of five books (and my most recent one entirely self-published), I haven't yet worked out how I feel about this or how to respond. My current compromise is to charge more on the DRM-free LeanPub.
Why charge more on a DRM free site? Do you think people buying from there are doing so that they can share the book illegally?
If someone wants to share the book illegally, I would imagine they'll just download it from one of the pirate mirrors out there and not bother paying you at all. My guess is you're probably just reducing the number of people willing to pay the price. Classic supply and demand curve against price.
FWIW, LeanPub for your book suggests $25, and the DRM-laden version is $13.50. That's quite the premium!