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).