Why wouldn't you? Your codebase (if you're a business) exists to make you money, people being able to copy some unknown portions of it without further license if they somehow legally get their hands on a copy of it seems entirely irrelevant.
PS. I think this is much less clear and much less settled law than you are suggesting.
It's more nuanced. If I even have a few lines I can prove are mine, those parts are copywritable in the same way Pride and Prejudice is public domain but pride and prejudice and zombies is copyrighted.
PS. I think this is much less clear and much less settled law than you are suggesting.