i can see why you are saying this .. seeing the world is full of scoundrels who make photographs of people and then they
plaster them all over the face book or the you tube or on bumper stickers or tee shirts or some other place where the public may find them
and the subjects may get upset they have been bamboozled into having their privacy disrupted ... but it isn't like this sort of thing is anything new ..
the other side of the coin is that if a photographer has a legitimate reason for photographing someone in public, and he/she
is honest with his/her subject about the reason for making the portraits, there is no reason not to ...
its no different than asking someone permission to photograph their red barn, or sheep or dog or harley .
i wish the OP lots of luck ...