I think the way I would go about arguing against it is that people on the street/sidewalk have no expectation of privacy. There are literally no access controls of any kind. Anyone can walk on the street; billionaires and homeless alike. There are no societal conventions that privacy is assured on the street and if you end up in someone else's picture it's your fault, not theirs.
Houses are not the street. They are private property. We do have a reasonable expectation of privacy there (NSA notwithstanding) and a part of privacy is access control. So the right of the owner of a house to control access to his house is fairly well understood and accepted even in the case where a house might be unlocked or a door left open.
The real question is this: Is the internet like the street or a house? The answer, in my opinion, is that "it depends" because websites can act both ways depending on how they are designed and implemented.
HN is basically a street in that it has no access controls to view content. Very nearly every page on HN can be accessed by the public (linked to or not) without being logged in. The URL of your comment is https://news.ycombinator.com/item?id=6434945 for which I didn't have to type in a password. What about comment https://news.ycombinator.com/item?id=6434944 or https://news.ycombinator.com/item?id=6434946? Should they be "protected" by virtue of them not being displayed on the webpage right now?
My credit union's website is a bit of public street and a lot of house. I can view their promotional materials without any authorization but in order to get to the good stuff I have to enter both a username and a password, then pass a captcha. That is an access control.
What is the case with the AT&T website? Did they do anything to secure the content with a technological access control like a username/password? Did they filter the service such that the webservice would only return an email address if it was accessed by the same MAC address of the iPad that was sold to the customer? No, they did none of these things. Their only "access control" was a user-agent string which isn't guaranteed ANYWHERE to be accurate.
EDIT: changed a couple of words