At the C2E2 comiXology panel it was confirmed that DC graphic novels are no longer being sold through Kindle Fire’s comiXology app, and CEO David Steinberger said it was intentional. However the reasons are hidden behind many layers of NDAs.
This is all odd because you can certainly still buy DC books via Amazon and their Kindle store. You can purchase tons of DC comics from comiXology, on the web and their IOS app. Apparently, you just can’t buy DC Comics through the Kindle Fire comiXology app (we don’t have a Kindle Fire handy to check).