Genuine questions here, not rhetorical or trying to imply anything with them.

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.