When the streaming service licenses a content, license will usually tell you where you are allowed to broadcast it. And producers for example rather than sell you a license that will allow you to stream globally, will sell you licenses based on regions. And if that region's license is already licensed to someone else, well you can't usually buy it since they would be made exclusive in the contract for some time or all of the duration of the license. That is why they are legally obligated or else it would be breach of contract.
So VPN services advertising that way, is not really illegal.