Well if it’s in a proprietary format, then sure. But if it’s not then no, they can’t do that. And it’s not like paper books haven’t been subjected to regional bans and text changes (see the recent roald Dahl news) so I don’t know what you’re getting at there. People can still import paper books regardless and people can get around digital restrictions through things like VPNs, torrents and what not also. There’s nothing stopping someone from sending a usb stick with an entire library of books into a country either and arguably a lot easier than sending one paperback as you could hide a usb in anything.