I’d love to see the contract those artists signed! I’m sure they did, or should have, gone in with their eyes wide open, and not sure what they expected from OpenAI (since clearly there wouldn’t have been any mention of payment in the contract), or whether OpenAI started asking them for a lot of work maybe (that’s the only thing I can think of that would give them the right to complain)?

They probably signed up for the purpose of doing this. People don't usually change their opinions that easily, especially a total 180 from "volunteering to support OpenAI" to directly the opposite.