Z:gnu-www-ja-gpl-2.0-faq--c4ab94-Our lawyers have told us that/en

Our lawyers have told us that to be in the best position to enforce the GPL in court against violators, we should keep the copyright status of the program as simple as possible. We do this by asking each contributor to either assign the copyright on his contribution to the FSF, or disclaim copyright on it and thus put it in the public domain.