You purchase the book written 30 years ago. It still is as it was 30 years ago. The writer is dead.
Tomorrow Amazon decides some terms don't conform to today's society and changes the book you already purchased, changes the text. Without the writer's or anyone else's permission. Maybe even if the writer is not dead, who knows.
If you had bought a device you control or the physical book, you'd have kept the original thing you purchased.
The day after Trump has a Trump moment and decides you should only read the Bible. Amazon removes the book from your device. Maybe you're refunded, maybe not, but you have no book.
You cannot read books from open formats with it but only Kindle's. Possibly you cannot add books to it, only read books purchased from Amazon (I think they still allow you to add books via USB but maybe tomorrow you have a "surprise firmware update").
Disclaimer : I don't know your device, your habits, your values, or how to jailbreak it. I'm not telling you you should jailbreak it.
I'm giving you arguments for, from now on, try to purchase things that don't lock you in, that give you as much control as possible, and if you have no such choice, try to limit their ability to control you and / or minimize your risk of losses however you can.