My guess is that when you sign up for either Anthropic or OpenAI, the terms of use specify you can't use their model for purpose A, B, C, D. For example, you can't use their model to try to build biological weapons, or to try to extort people, etc. Most likely there is language there that you can't use their models to train other models. It's as simple as that. You agree to those terms of use, or you don't use their models.