> Some cloud providers charge a round zero for intra-region traffic

That's likely a selling point to attract people to the competition. "Look, our bandwidth is free!"

Also there is quite a lot of cost for internal, inter-zone, traffic. You need all the routing equipment and the fiber links for that. And such traffic does not go through the same links as the public one, because you'd be paying the cost of public bandwidth trough transits or peerings. You want your own backbone, but this has a cost that needs to be amortized. Furthermore, properly designing such a network has a cost in engineers.