I need to read up at Flos Carmeli and Zippy Catholic, reread the blessed John Paul II's Letter to Artists, and maybe do some thinking, before I figure out what if anything I have to add to the "justice for artists" discussion. For now, let me just state a few brief impressions:
First, Steven's terrible experience of having a poem stolen and changed against his will strikes me as fundamentally a matter of verbal, not artistic, injury. It was wrong, not because of the inviolable integrity of Steven's poem-as-poem, but because it misrepresents the, to use an utterly inartful term, speech act he encompassed in the poem. It would have been equally bad had it been done to a transcript of a speech Steven gave.
Second, and relatedly, I will surprise no one by revealing that I suspect the old scholastic distinction between prudence as right reasoning about a thing to be done and art as right reasoning about a thing to be made will turn out to be helpful. (It's related to the first point since a speech is a matter of prudence, while a poem is a matter of art.)
Finally -- and I may have made this point before -- some of the complaints (such as about the Disneyfication of Kipling and Milne) amount to complaining that messing about with a work of art resulted in an inferior work of art. But if the thesis is that you ought never mess about with a work of art, the reason can't be because of the inferiority of the result unless the result is always and everywhere inferior. Otherwise you're just complaining about individual examples, not arguing for a general principle.