In the olden days of sail (no I'm not kidding) it would cost a lot to ship a significant number of books around the world. So the rights to publish a book might be sold by a British company to someone in the US or Australia.
Then there's the language problem. The UK company may not feel competent to publish a book in Polish, or some other language. So the rights get sold to a Polish publisher, and they do the translating and printing.
I think that's how international book rights began.
The publisher wants to sell the rights to publish a book in New Zealand or Australia. They DON'T want to ship it to New Zealand or Australia. Remember, if a book stays on the shelf too long the store wants to send it back to the publisher. Ripping the front cover off, and sending
that back, was a cheaper solution than sending the whole book back to the publisher.
Next comes the lawyers.
If a UK ebook seller can sell you an ebook in New Zealand, why would a New Zealand publisher buy the rights to publish that book? So the original publisher and the New Zealand publisher want to prevent that.
This is mostly a WAG. But it seems to make some sense to me.
The music industry didn't get straightened out until there was enough piracy to make them
want to fix it by removing the DRM and selling it to anyone with the money.
Maybe we should go back in history with Abbie Hoffman's book titled "
steal this book". Just a thought.