Yet somehow they still sell these books on Amazon?

Does it have that much better security?

Piracy is a customer service issue. Sell me your product in the format I want or I'll find a distributer who will. It's that simple.

Actually, I think this might teach some readers a valuable lesson about DRM.

Uh yeah real smart...
Publishers are pulling the Kindle versions of those titles, that may be the most difficult to copy and distribute. Those are the versions that a "regular person"* would most likely wait months in a queue, read during the three week checkout, and return un-copied. In the meantime, the publishers are leaving the pdf & epub versions that pirates use to make their versions.

*someone who owns a kindle or ipad and lacks piracy skills or the time to break codes