Why is it a bug? If other parts of the generated illustration are similarly taken from an artist, why not the signature as well? Why is a signature crossing the line but the rest of the image isn't?
I don't even believe too much in Intellectual Property but the signature amounts to a false representation, rather than a resemblance. The pixels are not special in the image file, but they're special when we look at them. This behavior should be trained out of the model.
Because it removes plausible deniability, duh
For the same reason I'm allowed to draw, paint, or write things very similar to what others have drawn, painted, or written but I have to sign my own name not theirs.