I was pleasantly suprised to find that, for the Kindle Direct Publishing platform, DRM is optional. That's right. You don't need DRM to sell ebooks on the Kindle Shop.
I suspect this might be the work of Cory Docterow, I believe I heard him mentioning that the did a deal with Amazon where they would publish his books without DRM. But I might be wrong.
I suspect another reason is that many self published authors want to use Creative Commons licenced works for book covers, and (depending on the licence) you would not be able to use DRM on a CC book.
That makes me wonder why they use it on the free, public domain books that Amazon snarfed from Project Gutenburg.
Can you link to examples? Random guess would be that they are not sold by Amazon directly but by third-party merchants on Amazon.
You're right. It was third parties self publishing through Amazon, not Amazon themselves. Amazon itself was simply the mechanism.