B&N aren't obligated to sell anything. In fact it's quite convenient to have a whitelist of books that are more probably worth reading than others. It is however troublesome that someone has the power to add or remove books from a common whitelist without respect to the will of the users of said whitelist. Which is why it's important for debatable removals to be reported on. This reporting will hopefully help to shed light on what motives and criteria control the inclusion and exclusion of books from said list.
What's really bothersome is the bundling of tangentially related services. Even if B&N may have developed an excellent payment system and handy e-readers, they should not be able to and allowed to seize control over the technology to greatly deter users from reading material not on their whitelist.
B&N should of course not be forced to advertise articles on cracking. But we must make sure that if they didn't advertise and distribute articles on cracking, it wouldn't matter unless everyone else on the Internet also decided to neither distribute nor read material on cracking.
Ideally B&N should publish a blacklist of reading material on cracking that you probably shouldn't read. Then whoever doesn't want to read material on cracking can filter that out and keep on with their lives believing that they're computer systems might be secure.