A Question About The Law

I recently posted a photo in the Retouching Forum of a sleeping man on a NYC subway train. I wanted some advice about photo editing.

One of the respondents asked me if I had gotten the guy's permission to take his photo

To the best of my knowledge, at least in the USA, one does not need permission to take someone's photo where there is no reasonable expectation of privacy. It's courteous to ask for permission, but one does not require permission.

Is this the way it is, pretty much?