Many people think it's wrong to significantly alter a work of art, not just because the result is aesthetically inferior, but because doing so wrongs the artist or is otherwise offensive. It's easy to see why, say, defacing a painting might be offensive. It's less obvious, though, why altering a work of music of literature might be bad. After all, a painting is a concrete, singular object; but novels and poems and symphonies are not. I can't ruin "Robinson Crusoe" or Beethoven's 5th in the way that I can a Matisse or a Van Gogh. Why should it seem problematic, for instance, to perform a piece of music in a manner deemed inauthentic, given that there's a sense in which "altering" or otherwise degrading the piece in its original, authentic form is just impossible.