I have seen dozens of debates about what is legal and what is not. What a person would think is moral and ethical and what is legal to do may be two diffrent things.

Talking candid street photography;
Personally, if I think in my mind does this bother this person, I choose to not take the picture out of respect for that person whether I know they approve or not. Not to say I am not tempted occasionaly.
Everyone will have their own moral compass on this, and I am not saying any one shouldn't be a street photographer they have the right to do it.
This is one reason I wouldn't ever be a street photographer, I avoid these kind of conflicts.

Of course I have always enjoyed messing with the street photographer in the same subtle way they try and get their pictures without the subject knowing, say move in to their background and give them half the peace sign or something similar. Of course I will smile for the picture, it is legal to do and the way I see it my moral compass has as much right as theirs. Maybe I am twisted this way, but if we are ever in a picture together watch out for horns.