It's also trivially easy with a few python scripts to strip the DRM from any kindle book you've purchased legally (think about it- the kindle has to be able to decrypt the book, and it's running on a pretty small chip). All you need to do is extract your decrypt key from the kindle, which turns out to be a function of the kindle's serial number.
I've done this for all the books I've bought for the kindle, to save a "just in case" version. It's also worth noting that the majority of piratebay books are pretty lousy OCR scans of books, with lots of markup and text errors. All the harry dresden books, for example, decided to be in a bold fond in the version I downloaded. Makes purchasing them a LOT more worthwhile (which I ended up doing for the first few, until I decided to give up on the series, but that's another story).
Still, I recognize that purchasing from amazon/bn/whoever is just supporting the business model of DRM, even if I strip out the DRM later. Would be nice to get somebody who didn't use platform lockin techniques, but that's probably unlikely in the near term.