If you believe in copyright then you don't believe in ethics^1 ;).
[1] Assuming you've actually thought about copyright from first principles—I think many people are too busy to have given it enough thought.
I don't know if that's true for everyone. If someone doesn't believe that an idea should be the property of the first person who thought of it for an innate moral reason, they might still believe that it's unethical to take that property away after it is guaranteed by the state. A utilitarian who is pro-copyright because it encourages creation and innovation might not be concerned with works that were published without that encouragement.