What if they called it appropriation art? (UPDATED)

So there's a new public artwork in China that seems to be a blatant ripoff of Anish Kapoor's famous "Cloud Gate" sculpture in Chicago.  It raises lots of interesting art law questions, one of which is captured in the title of this post:  what if we think of this as a work of appropriation art?  Remember the open letter Patrick Cariou wrote regarding Richard Prince's use of his work:

"I urge you to stand by my side and fight plagiarism. I feel compelled to ask what other businesses and innovators ... have had their copyrighted material stolen in a similar way? ... Creativity in all walks of life is hard won. It is incorrect to accept that we should allow for it to be undermined or stolen and therefore give it little or no value. ... We cannot let this happen."

Wait, that wasn't Cariou at all.  That was Kapoor, in a letter to Chicago Mayor Rahm Emanuel, who had responded to the news thusly:  "Imitation is the greatest form of flattery is what I would say. And if you want to see original artwork ... like the Bean, you come to Chicago."

So the first question about the case (and I hope to have more) is:  how do we distinguish this from other cases of appropriation, where the conventional wisdom seems to run in the other direction? Do the SuicideGirls count as other innovators who have had their copyrighted material stolen in a similar way?  Is it correct to accept that we should allow their creativity to be stolen?  Can we let that happen?

I'm not suggesting the two cases are the same.  I'm asking how we account for the difference. What is the theory, the principle, that makes the one case okay and the other not?

UPDATE:  Here's one attempt at an answer, from Nicholas O'Donnell.