I assumed that that was clear from the context, but let me rephrase it then:

"being made available DRM-free on Amazon" (and I'd narrow that down to "primarily/only on Amazon")

Of course public domain books are DRM free but I'm getting those from Gutenberg, not Amazon. Likewise, the copyleft books I'll most likely download from their own homepages, not Amazon.

I'm aware that DRM free media exists, including for currently copyrighted content that Amazon distributes ;)