Not even Amazon forces publishers to use it! There are DRM-free Kindle books available.
Unfortunately, I don't know of a way to explicitly search for DRM-free titles, but the description for these usually has a sentence like "made available DRM-free at the publisher's request", or is lacking the reference to "use limited to x devices simultaneously".
This is why I not only favor purchasing DRM-free media, but also keep a local archive of all such purchases.
$ ls /mnt/comixology | wc -l
239
(This can be a bit difficult for larger‐sized media, such as video games, but I do it anyway—my GOG archive is over two terabytes.)
I also have a small collection of DRM‐free music I had purchased and downloaded from Amazon that no longer seems to be accessible online. Certainly I have no plans to purchase anything digital from them in the future.
Including in KDP. There's a tickbox for whether you want DRM or not. Personally I'd never add DRM to mine because it's so trivial to strip anyway and just an inconvenience.
If Amazon doesn't demand it, then it must be the book publishers that are actually demanding it. So it's not just big bad Amazon after all. That's interesting.
Main reason I mind is because DRM a) creates "artificial scarcity" of an effectively "infinitely" and freely duplicate-able resource (digital bits of data) mostly to fulfill the bottomless greed-hole of corporate entities (rather than to enrich the original creator for their part in it), and b) (and this is the main reason I hate DRM) because it gives some bottomless pit of greed corporate entity the ability to arbitrarily deprive me of something I paid for, or restrict my ability to use / access it at some unknown point in time down the road, forcing me to maybe have to pay again for something I already paid for. (Nope. Not gonna do that. If I'm backed into that corner, I'll choose to "fly the Jolly Roger" and acquire a DRM-free copy some other way.)
Given the option to support content creators in some way that doesn't screw me over as a content "consumer", I'ma prefer that option all day long, every day; However, too many corporate entities these days would rather not offer that option, if they think they can find a way to make you pay every single time you lay eyeballs on something… (That is the ultimate goal of some of 'em, to be certain.)
I see it less as bottomless greed, and much more as a fear or inability to adapt to a new situation, desperately clinging to the past via technological means instead.
The music industry was the same, initially – and maybe MP3s would still be sold with DRM by default if it wasn't for Steve Jobs and the iTunes store back in the day.
There's hope, though: With very few exceptions, German e-books are also sold without DRM these days.