The law preserves for the creator of a photograph (or of a painting, for that matter) the right to both reproduce the work and to create other, derivative works from it. Period. (See http://www.copyright.gov/circs/circ1.html.) No one else has this right without explicit permission from the creator.
Although I'm a dedicated plein air painter, I admit I do paint from photos now and then. However, these photos are my own. Why don't I paint from photographs other people have taken? (Other than the legal issue, of course.) Photographs that aren't my own don't have enough meaning for me. They may be pretty, but I can't feel the moment. For me, my photos evoke a lot more than what shows in the photograph. They evoke a whole world of sensation: the warm wind on my cheek, the fragrance of the sea, the cries of the seagulls. My photos may evoke a sense of early-morning optimism, noon's calm repose, or the moodiness of a fin de siècle evening - a sense I may want to bring to my painting. It's hard to find this in someone else's photograph, and even if I do, I still don't know what colours the shadows really were. I'll remember the shadow colours when I see my photo.
If you know a painter who paints from someone else's photos (or paintings), explain to them that not only are they doing the creator of the work harm -- every illegal copy that gets sold is one less that the creator can sell and is thus bread stolen from his mouth -- but they are doing themselves a disservice.
But, as you know, I really don't like painting from photos, anyway. One becomes a better painter by painting from life, not from photos. I leave you with this recent piece I did of a lovely house in Wilson's Beach, here on Campobello Island.