Street photography can be very powerful, especially to visually explain the relationship members of a community, or those who are effectively outcasts, have with each other and the larger society in which they exist.
Intent seems to be an important part of the moral discussion. Wanting to tell a full story of travels, rather than just what a tourist bureau would approve, I would include photos of the very poor and how they live. Of course a photojournalist documenting a location, the plight of particular segments of society, or contradictions within a society has a duty to be honest with choice of subjects.
Using photos of one group of people as a warning to others can also be morally perplexing. For example, to attempt to prevent a self-destructive behavior, would it be ok to go into a hospital and, without the subjects' knowledge, take photos of people suffering from drug abuse, STD's, smoking, careless driving, or neglected diabetes? If not, would your answer be based on their location, that a hospital is a special place for privacy? Then what of the photographer whose motive is to save lives of young people by taking pictures of drug addicts languishing in a public park or on a sidewalk? Is this ok because the suffering occurs in a public space? Is it ok to take pictures of a toothless, lesion-riddled amphetamine addict who happens to be dozing with mouth open if the purpose is to "scare straight," for example? (In other words to use the sad images to frighten the viewer into avoiding destructive behavior.)
Finally, even trickier moral issues do arise for some photographers and their audience when the purpose of a photo is purely artistic, where the subject becomes no more important to the photographer than a bird or an interesting found still-life. I do not feel comfortable with such work even when the subject has been paid a nominal fee, especially when the "artiste" receives accolades for images made with thousands of dollars worth of equipment and a "tip" worth the price of a cup of coffee to the desperate subject.
Also I do see street-photography as a sport (akin to birds-in-flight) as a somewhat coldblooded objectification of human beings.
No, I don't think pleasuring the egos of the artist, the viewer, or the curator to justify using unknowing (and perhaps unwilling) human subjects.
On the other hand, in most places in the USA, street photography is completely legal, and if remaining within the law satisfies ones conscience, who are we to condemn?
Surapon: Glad to see you back and, once again, posting provocatively!