The issue isn’t the final, individual art pieces, it’s the scale. An AI can produce sub-par art quickly enough to threaten the livelyhood of artists, especially now that there is far too much art for anyone to consume and appreciate. AI art can win attention via spam, drowning out human artists.
People were also a lot more open to their data being used by machine learning because it was used in universally appreciable tasks like image classification or image upscaling; tasks no human would want to do manually and which threatens nobody.
The difference today is not the data used, but the threat from the use-case. Or, more accurately, people don’t mind their data being used if they know the outcome is of universal benefit.