Heres a real life example of how things can go wrong when someone takes your photograph... and in this case, the person knew her photograph was being taken!
Virgin sued for using teen's photo - Technology - smh.com.au
The short version is that a girl smiled for a camera at a church barbeque. The photographer then posted the photos of the day on Flickr to share with friends and family. An ad agency found the photos and took the picture of the girl and used it in a billboard campaign in Australia with the slogan 'Free text.. Virgin to Virgin'. (the ad was for Virgin Mobile). The ad was quickly seen by the girls brother who reported home!
The underlying issue is FlickR... The youth councellor you snapped the photo checked a box when she created her Flickr account that put her photos in an area that gave ad agencies permission to use them.
So now, you're reading a book or eating your lunch at a mall, some photographer snaps your photo and posts it to Flickr and then you're on a billboard ad for STD drugs or some other embarrassing product.
It's just not right!