IANAL, but It is worth noting here that DeepSeek has explicitly consented to a license that doesn't allow them to do this. That is a condition of using the Chat GPT and the OpenAI API.
Even if the courts affirm that there's a fair use defence for AI training, DeepSeek may still be in the wrong here, not because of copyright infringement, but because of a breach of contract.
I don't think OpenAI would have much of a problem if you train your model on data scraped from the internet, some of which incidentally ends up being generated by Chat GPT.
Compare this to training AI models on Kindle Books randomly scraped off the internet, versus making a Kindle account, agreeing to the Kindle ToS, buying some books, breaking Amazon's DRM and then training your AI on that. What DeepSeek did is more analogous to the latter than the former.