AKA, the original whispersync.

Yup, this was once a thing - you didn't need wifi for sync or downloading books at all.

To save money they could come with LoRA radios and sync when the opportunity arrises to a LoRA gateway, including meshing with each other to aggregate data to increase the likeliness of encountering a gateway. LoRA modules are pretty cheap.

https://www.thethingsnetwork.org/

...which would require a valid SIM. So just don't add one. If the device comes with a pre-inserted/hardwired/virtual SIM, well... several countries in the world require KYC-style registration of the SIM owner before networks are allowed to activate the SIM, so there'd still be an opt-out path for the user in such countries.

eta: My point being: Now you're in a twisty little maze full of corner cases, all different. Not the sort of thing much loved by Amazon (or any of the GRAFT).

Is that Kindle that promiscuous that it will literally connect to any open AP without prompting?