I don't know enough about how GTP-3 and ML work to really answer this, but I think I'd be fine with what you're saying if I understand the question. If they provided (and charged) for the infrastructure, but the model was FOSS and community-driven, it would be less icky I think.
I just don't like the idea of taking people's work (without asking or checking licenses) and then selling it back to them. It'd be like if Stack Overflow decided to start charging to see answers and not asking or giving a split to the person who gave the answer. I realize they aren't just copy/pasting so not a perfect parallel, but still.
How would you feel if they just provided the software without the model, assuming you could train it yourself on open-source code in an instant?