> The difference is that people who write open source code or release art publicly on the internet from their comfortable air conditioned offices voluntarily chose to give away their work for free

That is not nearly the extent of AI training data (e.g. OpenAI training its image models on Studio Ghibli art). But if by "gave their work away for free" you mean "allowed others to make [proprietary] derivative works", then that is in many cases simply not true (e.g. GPL software, or artists who publish work protected by copyright).