I think that we would have to rely on the photographer’s discretion. There can be no expectation of privacy when you are in a ”public” environment. As @mollyc stated earlier, if someone asks not to have their picture taken—when it’s obvious that they will be included in a photo, then the photographer can either acquiesce or ignore.I think something needs to be considered, too, in exactly what the situation is where the photographer is shooting..... Is it a public event? Is it a massive fire in the neighborhood? Is it something happening in what is indeed truly "public space" and where no expectations of privacy would exist? Or is the photographer shooting images of strangers shopping in a store, browsing through a mall, eating in a restaurant or relaxing in a bar where most people are just doing their own thing and aren't expecting some stranger to be taking their picture at all?
It is not up to us to question someone’s ethical standards when they are creating a visual expression of what they are viewing at that moment.