I agree with the OP who says that you should read the terms of any agreement.
But if you want to know why you should read things for yourself and not, like, just randomly accept things other people say, the last comment is a perfect illustration.
So hereâs an example of how you read a contract properly.
Amazonâs T&C are here. https://kdp-eu.amazon.com/agreement?token=eyJjbGllbnRJZCI6ImtpbmRsZV9kaXJlY3RfcHVibGlzaGluZyIsImRvY3VtZW50SWQiOiJrZHAiLCJjYW5jZWxVcmwiOiJodHRwczovL2tkcC5hbWF6b24uY29tIiwiY2xpZW50TG9jYWxlIjoiTkEiLCJkaXNwbGF5UGFyYW1zIjpudWxsfQ%7CeyJtYXRlcmlhbFNlcmlhbCI6MSwiaG1hYyI6IjNVTU9oejdYTENWY1dsMUZEWWQwYnRZN3FNR1FxOEJxVHpyaWhYS2dGVWM9IiwianNvbkhtYWMiOnRydWUsInR5cGUiOiJSRUFEX09OTFkiLCJhY2NlcHQiOmZhbHNlfQ&language=en_US
Does Amazon contractually own your work in perpetuity? Well, the things you need to look for are the term of the agreement (legalese for âhow long this lastsâ and then the rights they ask for.
Hereâs the term of the agreement:
3 Term and Termination
The term of this Agreement will begin upon your acceptance of it and will continue until it is terminated by us or by you... You are entitled to terminate at any time by providing us notice of termination, in which event we will cease selling your Digital Books within 5 business days from the date you provide us notice of termination.
In other words, this agreement is VERY easy to terminate--you just tell them you want out and you are out in five days. This is easier to get out of than your standard cable contract.
(Note that the section on KDP Select has separate terms/right grants, but thatâs optional from what I understand.)
5.5 Grant of Rights. You grant to each Amazon party, throughout the term of this Agreement, a nonexclusive, irrevocable, right and license to distribute Digital Books, directly and through third-party distributors, in all digital formats by all digital distribution means available.Â
(emphasis mine). Thereâs more (and Iâll get to that), but that is most definitely a time-limited agreement--the rights you grant are for the term of the agreement. Remember the term of the agreement? Yep. You can cancel in five days. So...not exactly forever-level theft ownership.
So the next section of the rights grant gets more specific, and Iâm going to snip most of it, but the question is... do any rights that Amazon asks for survive the term of the agreement?
I read the whole section, but Iâm only going to copy the rights that survive the term of the agreement:
This right includes, without limitation, the right to:... (c) permit customers to "store" Digital Books that they have purchased from us on servers ("Virtual Storage") and to access and re-download such Digital Books from Virtual Storage from time to time both during and after the term of this Agreement;Â
Thatâs it, thatâs the only right youâre granting Amazon that survives the term of the agreement--the right for customers who have paid you money for your  work to be able to read that work as if it were a real book on their shelves.
Iâm guessing 99.9% of creators are going to think itâs totally cool that people who bought a thing get to keep enjoying the thing they bought even after youâre no longer working with the person who sold it to them.Â
If youâre in the 0.1% of people out there who think that itâs theft for consumers to enjoy something theyâve already purchased, then definitely donât publish through Amazon.
But thatâs why you should read your own contracts--because some other random person out there is...not guaranteed to be right about what they read.