Personal docs don’t sync page progress
Library checkout process has a sign out button right next to the borrow button
It is all subtle nudges that feel designed to keep you in ecosystem.
From what I recall (though I haven't used a Kindle in a few years), email-imported .mobi files show up in your library and sync progress just fine. It's just other filetypes that don't sync.
Hmm, I never considered that because I mostly just send-to-kindle-by-email and then read those docs from only one kindle device. How many devices do you read on?
I think it is a liability issue. Amazon probably decided that someone might sue them if their servers were involved in helping the user read possibly pirated books.
Which is stupid, and I think no one should ever be sued for this, but the Amazon legal department probably had the final say in this.