> I often have to put in an extra edit to erase the false signature.

Given that they can reliably do this, I'd think it should be trivial for the harness to automatically insert a "review the image for anything that looks like an artist's signature or other blatant indicator of plagiarism, and fix it" pass.

Yes, but that could double the cost by default! Which is probably why they don't. On reflection, coding bots do implement something like this with the verification tic they seem to have.