Supposedly, copying art ruins the aesthetic value. The more times it is duplicated, the more it ruins its authenticity. Right? Well take the example of movies that have been made based off of books. They recreate the entire book, using actors that resemble the characters almost exactly. Then they recreate the scenes and the plot, etc. to the point of complete copying (the good ones anyway).
Now take the example of video games or plastic toys based off of comic books, or movies. Again, they recreate the characters, the scenes, or at least the concept. So if directors and producers can recreate things and make different versions of it, and it can be socially acceptable (sometimes), then what is the difference when other artists recreate paintings. Most of the time they do it to preserve something, or to interpret it differently. Same goes for when movies or video games are copied off of something else. The do it to preserve something or to recreate it. Like how the classic black and white movies are re-made into modern times. . .