Just like how it is a bad idea to train github copilot on copyrighted code, or how the same company that made Stable Diffusion promised that they will not use copyrighted music for training (because they're scared of the music industry), copyrighted art should not be used on training sets without permission.
This is going to cause a lawsuit somewhere down the line, even if images only contribute a few bits each, signatures are still seen and this could make an argument in a court case.
It would be fair to everyone to only use public domain material for training sets.