... My understanding is that while people are free to photograph private property, they are not free to use it commercially without permission. Am I wrong about this? How does this work?
The whole misunderstanding is in the word "commercially." From the standpoint of copyright legislation, it is defined much more narrowly than in common parlance. It means "for advertising," not just making money, directly or indirectly, off of it. In your example, those photos are offered for free by their users as a public service to other users of maps. And even if those photographers were paid by Google, it still would not be considered "commercial use" for copyright purposes. If, however, Google were to place an ad in newspapers, TV, or web, advertising their maps feature and use one of those photographs, than it would be a commercial use and subject to copyright infringement of both, photographer and property owner's, rights.
