I think you've got two questions here:

1) should a photograph ever exploit someone eg. the homeless, or the man on the tracks.?

2) is it right to take pictures of people without their permission?

I think the answers are quite obvious (to me at least): 1) No, never and 2) Yes, as long as the law of the land allows it (and even sometimes when it doesn't).

I've taken pictures of homeless people, but only ever shown two of them:

This one, the chap called me over, and said he wanted his picture taken, but that it'd cost me a pound. I have no idea if he was/is really homeless, but I accepted his offer and gave him the change in my pocket (~£1.50). If I wasn't in a hurry, I'd have rather asked if he was hungry and bought him a sandwich and a coffee, but I do not felt I was exploiting him in any way at all.



And this one, was definitely not to exploit the homeless-person. It was more an observation of the bloke walking past and looking. People normally ignore the homeless, but this guy didn't, he looked at every one of them with concern, and bought a sandwich for one of them that was awake.



As for the second question. I fully believe in my right to take photographs of the public. And luckily, the UK law permits me to do so in any public location. I make no money from my art, and I respect anyone who chooses not to take pictures of the public, but personally I don't find anything wrong with it as long as it is done in good faith.