Moreover, the subjects of photos can sue you for invasion of privacy -- or, depending on how you use the image, slander. Property owners can sue you for trespassing. And, in some parts of some countries, there is a "right of publicity":
http://www.publaw.com/rightpriv.html
... which prevents you from (e.g.) taking a photo of a well-known person, Photoshopping your product into their hands, and using the result for commercial purposes.
The end result is that professional photographers, videographers, and filmmakers try to get signed release forms from every recognizable person who appears in a shot that is to be used for a commercial purpose. [2] The documented existence of those release forms is a major reason why stock photos cost more than your brother's photographs of random pedestrians on the street.
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[1] Have I mentioned in this thread that I'm not a lawyer? I'm not a lawyer.