Isn't photography all about voyeurism? About taking home what you see?
Courtesy and discretion is all very nice, but if you are a photographer, what you do is making photographs. Sometimes courtesy and discretion are misplaced. Sometimes the whole point of the photograph is that it is discourteous, that it is indiscreet. Should HCB have just put away his camera when he saw the kid with the wine bottles? Should the soldiers who liberated the concentration camps have refrained from documenting what they saw? How about war photography? Was it wrong for Eugene Smith to shoot a mother in the bath with her deformed child? Is it unseemly to photograph fat people, or weird people? Should Diane Arbus be censored?
I don't have all that much experience at candid photography, but I think a smile, a nod and maybe a thank you should suffice, after you have taken the shot. And if you snap something funny, the people in the shot may be embarrassed, but that doesn't really count. A good joke about the pope and the president is a good joke, wether they are embarrassed or not.
What strikes me is that those who call for discretion and courtesy must have had a polite upbringing, whereas those who react badly to photographers usually have no manners to speak of (at least from what I can glean of discussions of this topic, and my meagre experience). If someone has bad manners, doesn't he deserve to be exposed in a photograph?
Restricting oneself from taking candids of children, sleepers or homeless people is just wrong. Homelessness is rising at such a fast rate, it is high time somebody did some serious documenting on this social phenomenon. I have a friend who is building a gigantic project called 'Sleepers on the train. Fascinating work. Sleepers may be funny, but they are always innocent. And what is so wrong about photographing children? Over-protective parents? Children are just great subjects : lively, unpredictable, and mostly a lot of fun.
What is wrong with us, that we should want to restrict ourselves in these silly ways. Next it'll be wrong to photograph clouds, for fear of offending the owner of a satellite that was overhead at that time.