Interesting point. I’m not sure that could be supported in court. (Again, IANAL).
I mean, it sort of depends. Is a portal more like a hot dog stand or more like a Yelp review? It would have to be more like a Yelp review or Niantic would owe licensing fees for every single “commercial” use of property, ie, every portal. And if it’s more like a Yelp review then I don’t think Niantic is responsible for how the user uses it.
My point about all property having an owner was that “private property” is largely irrelevant to the question of licensing. If a license, or permission, or royalties are required, then they are likely to be required of all property regardless of the owner.