Earlier quoted context omitted.
> you have [to] send [books] through [Amazon's] servers. No, you can sideload books using USB mass storage. It's pretty easy. Kindle Paperwhite is still a great experience even without using the Amazon book ecosystem.
You are correct, you can sideload, but as soon as you open them in your Kindle, they get an Amazon-DRM; so you can't read the very same files on another e-reader. And - as soon as you go online with your Kindle - said DRM is checked and all non Amazon books deleted. At least, that was the case 10 years ago: I still own a Paperwhite 1st Gen which is now basically defunct. I switched to a Poke 5P (Onyx) and was surpris…
Not true and never has been. The Kindle will make no changes to sideloaded files.