It's "art" devoid of emotion or sentiment. It's removing humanity from one of the purest forms of human expression. It's a reduction of art into pure aesthetics. There is no meaning, there is no intention, there is no connection to be made. There's certainly no message or politics (and we all know how icky this weird push for "apolitical" art is, and how it benefits the ruling class).
There's also an underlying sense that aiding in training AI to produce art is to participate in the destruction of art as a human pursuit. Already artists are rarely compensated for their work, and society is discouraged from believing that "artist" is a worthwhile or valuable pursuit, while at the same time we are all encouraged to consume "art" at an ever-increasing rate. This is just the next step in devaluing the human labour of creation by automating something that truly should never be automated, so that a few years from now corporations can sell us all perpetually-trademarked computer generated media and completely remove artists from the equation altogether.