The process of appropriating in the art world has existed as far back as I can remember. There has always been artist that have used outside elements in their work.
Di Vinci was appropriating images from science, biology, math and geometry books. Piccaso used photographs from news paper clipping in some of his paintings. Is architecture a form of art? If so, look at all the photographers that take pictures of it. So we cannot call there work original?
We photograph the natural landscape of this earth that God created. Are we just appropriating and stealing what he has done?
Of course, Levine caused alot of legal controversy when she did that work back in the 60's-70's, but she was able to defend her work well and people accepted it. It was partial because she was not trying to hide the fact that they were copies of images. Just by looking at her prints, you could tell they were not original and she was also using the titles like "After Edward Weston" which made it okay.